MS-PS2-2: Planning Investigations of Force, Mass and Motion
In plain English: MS-PS2-2 is the NGSS middle school performance expectation that asks students to plan an investigation showing that an object's change in motion depends on the sum of the forces on it and on its mass. Students change one variable at a time, compare balanced and unbalanced forces, and state their units and frame of reference. It is usually taught in grade 7 or 8 physical science.
Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.
Clarification Statement: Emphasis is on balanced (Newton's First Law) and unbalanced forces in a system, qualitative comparisons of forces, mass and changes in motion (Newton's Second Law), frame of reference, and specification of units.
Assessment Boundary: Assessment is limited to forces and changes in motion in one-dimension in an inertial reference frame and to change in one variable at a time. Assessment does not include the use of trigonometry.
Next Generation Science Standards · Disciplinary Core Idea: Motion and Stability: Forces and Interactions · Official standard
Students plan and carry out investigations that show how an object's change in motion depends on two things: the sum of the forces on it and its mass. A change in motion means speeding up, slowing down or changing direction. Students add forces along a line, tell balanced forces (sum of zero, so the motion stays the same) from unbalanced forces (the motion changes), and compare how objects of different mass respond to the same sum of forces. Planning is the main practice: students name the variable they change, the variable they measure and the variables they keep the same, choose units, and say what the motion is measured relative to.
The NGSS Clarification Statement says: "Emphasis is on balanced (Newton's First Law) and unbalanced forces in a system, qualitative comparisons of forces, mass and changes in motion (Newton's Second Law), frame of reference, and specification of units." The Assessment Boundary says: "Assessment is limited to forces and changes in motion in one-dimension in an inertial reference frame and to change in one variable at a time. Assessment does not include the use of trigonometry." Every force on this page acts along one straight line, every investigation changes one variable at a time, and all motion is measured relative to the ground or the classroom floor. All data sets are invented for teaching and labeled that way.
Learning Objectives
By the end of this lesson, students will be able to:
Find the sum of the forces on an object when the forces act along one line, using + and - for the two directions
Tell balanced forces from unbalanced forces and predict whether an object's motion will change
Plan an investigation that changes one variable at a time (the sum of the forces or the mass) and names the measured and controlled variables, the units and the number of trials
Use data to describe how a larger sum of forces and a larger mass each affect an object's change in motion
Describe motion relative to a stated frame of reference
Prior Knowledge Required
Students should already be comfortable with:
Pushes and pulls of different strengths and directions change how an object moves 3-PS2-1
Force pairs in Newton's third law MS-PS2-1
Measuring distance with a meter stick and time with a stopwatch
Recognizing proportional relationships in tables and graphs 7.RP.A.2
Slide an empty plastic box across a smooth floor, then a box full of water bottles, with what looks like the same push. Ask:
Warm-Up Prompt
"The full box barely moved. Was that because it is heavier, because the push was different, or both? How could you set up a fair test to find out?"
List students' ideas on the board. Many students suggest changing the push and the load at the same time; ask how they would know which change caused the result. Keep the list: students turn it into a plan later.
Direct Instruction15-20 minutes
Key words. A force is a push or a pull, measured in newtons (N). Friction is a force between two surfaces that rub, and it acts against the sliding. When several forces act along one line, choose one direction as + and the other as -. Their total is the sum of the forces (also called the net force). Mass is the amount of matter in an object, measured in kilograms (kg).
Balanced and unbalanced forces. If the sum is zero, the forces are balanced and the motion does not change: an object at rest stays at rest, and a moving object keeps the same speed in the same direction (Newton's first law). If the sum is not zero, the forces are unbalanced and the motion changes in the direction of the sum. A larger sum gives a larger change in motion, and the same sum gives a smaller change to an object with more mass (Newton's second law). Diagram 1 shows both cases.
Frame of reference. A frame of reference is the place or object you measure motion from. In this lesson we measure all motion relative to the classroom floor or the ground.
Planning words. The independent variable is the one thing you change on purpose. The dependent variable is what you measure. Controlled variables are kept the same. A trial is one run of the test.
Adding forces along a line
Two students push a heavy box to the right with 30 N and 20 N. Friction on the box is 15 N to the left. Call right +.
Result: Sum = +30 + 20 - 15 = +35 N: 35 N to the right. The forces are unbalanced, so the box speeds up to the right.
Balanced forces, then unbalanced (vertical)
An 800 kg elevator car with riders hangs from its cable. Its weight is 7,840 N down and the cable pulls 7,840 N up. Then the motor makes the cable pull 8,640 N up.
Result: First: sum = 7,840 - 7,840 = 0 N. The car is at rest or moving at a steady speed. Then: sum = 8,640 - 7,840 = 800 N up, so the forces are unbalanced and the car speeds up upward.
Same force, different masses
A delivery truck has a mass of 5,000 kg when empty and 10,000 kg when loaded. In both cases the sum of the forces on it is 5,000 N forward for 10 s, starting from rest. In an invented test, the empty truck reaches 10 m/s.
Result: Loaded: twice the mass with the same sum of forces, so half the change in speed: about 5 m/s after 10 s. More mass, smaller change in motion.
Planning one variable at a time
Question: does a larger sum of forces make a 2.0 kg storage box on furniture sliders speed up more? The box is pulled with a spring scale, which shows the size of a pull in newtons.
Result: Independent variable: the sum of the forces (pull minus friction), in N. Dependent variable: distance traveled in the first 2.0 s from rest, in m. Controlled: the box's mass, the floor, the start from rest, the timing method. Three trials at each force.
Frame of reference
A student stands on an airport moving walkway that moves at 1.0 m/s and holds a ball. Then she walks forward on the walkway at 1.5 m/s.
Result: Standing: the ball moves at 0 m/s relative to the walkway and 1.0 m/s relative to the floor. Walking: 1.5 m/s relative to the walkway and 1.0 + 1.5 = 2.5 m/s relative to the floor. Always say which frame you use.
After each example, ask: "What is the sum of the forces? Is it balanced? What would you change, and what would you keep the same, to test this?"
Guided Practice15 minutes
Pairs analyze invented data from an online forces-and-motion simulation with friction turned off. A 200 kg crate starts at rest. In each trial, one or two people push for 2.0 s, and the simulation reports the crate's speed at the end. Call right +.
Guided practice data (simulation, invented): 200 kg crate, friction off
Trial
Push to the right (N)
Push to the left (N)
Speed after 2.0 s (m/s)
1
100
0
1.0
2
100
50
0.5
3
100
100
0
4
150
50
1.0
5
200
0
2.0
Add a column for the sum of the forces and fill it in for each trial.
Trials 1 and 4 use different pushes. Compare their sums and their results. What does this tell you about what the crate "responds to"?
Plot the speed after 2.0 s against the sum of the forces, then compare your graph with Diagram 2.
Which variable was changed in this investigation, and which one stayed the same? Write one sentence about a new set of trials that would test the effect of mass.
Independent Practice10-15 minutes
Each student critiques three plans for testing the storage box and rewrites the weakest one.
Plan A: "I will pull the empty box with 3 N, then put two water bottles in it and pull with 5 N, and compare the distances."
Plan B: "I will pull the box with different forces and see what happens."
Plan C: "I will keep the box at 2.0 kg and measure the friction first. I will pull so the sum of the forces is 0.5, 1.0 and 1.5 N, measure the distance in the first 2.0 s from rest against tape marks on the floor, and run three trials each."
Students explain that Plan A changes two variables at once (force and mass), Plan B names no measurement, units or controlled variables, and Plan C changes only the sum of the forces. Each student then writes a Plan D that tests the effect of mass, keeping the sum of the forces the same.
Closure5 minutes
Exit ticket: "In a tug-of-war, Team A pulls the rope to the left with 420 N and Team B pulls to the right with 450 N. What is the sum of the forces on the rope, and how does its motion change? Then name one variable you would keep the same if you wanted to test the effect of mass on a sled." (30 N to the right, so the rope starts moving and speeds up toward Team B; keep the sum of the forces the same.) Return to the warm-up list and mark which ideas would make a fair test.
Differentiation Strategies
For Struggling Students
Give a planning frame with labeled boxes: question, independent variable (with units), dependent variable (with units), controlled variables, number of trials
Draw forces as arrows on grid paper, one square per newton, so students can count squares to find the sum
Start with forces in one direction only before adding a force in the opposite direction
For Advanced Students
Ask students to explain why friction must be measured again each time mass is added to the box, if the sum of the forces is to stay the same
Have students plan how to test whether the dependent variable they chose (distance in 2.0 s) really doubles when the speed-up doubles, using video
Ask students to compare two frames of reference for the same motion and explain why scientists usually measure from the ground
Assessment Guidance
What to Look For
Check that students subtract opposite forces instead of adding every number, and that they treat balanced forces as "no change in motion," not "no motion." In plans, look for exactly one independent variable, a measurable dependent variable with units, at least two controlled variables and repeated trials. Listen for students who think a moving object needs an unbalanced force to keep moving, and for students who forget to say what motion is measured relative to.
02
Classroom Activities
3 Activities
1
Slider Box Lab: Changing Force, Then Mass
30 minGroups of 3-4
Groups pull a plastic storage box on four furniture sliders across a smooth floor with a 10 N spring scale. In Part A they change only the sum of the forces; in Part B they change only the mass. They measure the distance the box travels in the first 2.0 s from rest.
Procedure
Put tape marks on the floor every 10 cm for 2 m. Find the friction: pull the box at a slow, steady speed and read the scale. At a steady speed the forces are balanced, so the reading equals the friction.
Part A (box at 2.0 kg): pull with 0.5, 1.0 and 1.5 N more than the friction, keeping the scale reading steady. Film from the side and read the distance at 2.0 s. Run three trials each.
Part B: add 1 L water bottles (1 kg each) to make 3.0 kg and 4.0 kg. Measure the friction again each time, then pull with 1.0 N more than the friction, so the sum of the forces stays at 1.0 N.
Safety: clear the path, walk (never run) beside the box, and nobody rides on it.
Sample Data (invented)
Friction: 2.0 N at 2.0 kg, 3.0 N at 3.0 kg, 4.0 N at 4.0 kg. Part A (2.0 kg): sum of forces 0.5, 1.0 and 1.5 N gave mean distances of 0.52, 0.97 and 1.49 m in 2.0 s. Part B (sum of forces 1.0 N): masses 2.0, 3.0 and 4.0 kg gave mean distances of 0.98, 0.69 and 0.50 m.
Discussion Questions
In Part B the scale reading got larger each time, yet the box went less far. Why is that not a contradiction?
In Part A, what happens to the distance when the sum of the forces doubles from 0.5 N to 1.0 N? In Part B, what happens when the mass doubles from 2.0 kg to 4.0 kg?
Why do we measure the friction at a steady speed?
Modification for Distance Learning
Students run the same two-part plan in an online forces-and-motion simulation, changing one variable at a time, and post their data tables and a one-paragraph conclusion.
2
Spring Scale Tug-of-War on a Roller Skate
15 minGroups of 4
A 1 kg bag of rice sits on a roller skate on the floor. Students hook spring scales to both ends and pull in opposite directions, alone or in pairs, to see when the forces are balanced and when they are not.
Procedure
Hook one or two 5 N scales to each end of the skate. Call right +.
For each trial, pull to the target readings, hold for 2 s if the skate stays still, and record the readings and what the skate does.
Safety: pull gently and slowly, keep feet clear of the skate, and never let go of a stretched scale.
Sample Data (invented)
Trial 1: 3.0 N left, 3.0 N right, skate stays still. Trial 2: 3.0 N left, 4.0 N right, skate rolls right and speeds up. Trial 3: 2.0 N and 2.0 N left (two scales), 4.0 N right, skate stays still. Trial 4: 2.0 N and 1.5 N left, 5.0 N right, skate rolls right and speeds up.
Discussion Questions
In which two trials were the forces balanced? Show the sums.
Trial 3 uses three scales and Trial 1 uses two. Why does the skate behave the same way?
Compare Trials 2 and 4. Which has the larger sum of forces, and how should the speed-up compare?
Challenge Variation
Start with the skate rolling slowly to the right, then apply balanced pulls of 3.0 N on each side. Predict and test: does the skate stop, speed up or keep rolling? (Friction in the wheels slowly stops it; ask students how they would show that friction is the cause.)
3
Plan-a-Test Stations
20 minPairs
Pairs rotate through four question cards. At each card they write a full plan on a planning sheet: question, independent variable, dependent variable with units, at least two controlled variables, number of trials, and frame of reference. Pairs then swap sheets and check each other's plan for one-variable-at-a-time.
Question Cards
Card 1: Does adding identical rubber bands, each stretched to the same mark, change how much a roller skate speeds up?
Card 2: Does the mass of a plastic puck change how fast it moves after the same flick from a ruler bent back to the same mark?
Card 3 (vertical): A helium balloon holds just enough paper clips to float in place. What happens to its motion when one paper clip is removed, and when two are removed?
Card 4 (frame of reference): How fast does a ball rolled down the aisle of a bus that is moving at a steady speed travel, as measured from inside the bus and from the sidewalk?
Discussion Questions
Which card changes the sum of the forces, and which changes the mass? Which one tests balanced forces?
On Card 3, what is the sum of the forces on the balloon while it floats in place?
On Card 4, why do the two observers report different speeds, and who is right?
Modification for Distance Learning
Post the four cards on a shared slide deck. Pairs type their plans on the slides, and the teacher picks two plans for the class to improve together.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Balanced and Unbalanced Forces on the Slider Box
Force arrows on the 2.0 kg storage box, drawn to scale. In A, the 2.0 N pull and the 2.0 N friction are balanced, so the box keeps its motion. In B, the 3.0 N pull and the 2.0 N friction leave a sum of 1.0 N to the right, so the box speeds up.
Diagram 2: Speed After 2.0 s Against the Sum of the Forces
The guided practice simulation trials for the 200 kg crate, drawn to scale. The points fall on a straight line through the origin: the speed gained is proportional to the sum of the forces. Trials 1 and 4 land on the same point because their sums are equal, even though the pushes are different.
04
Homework Assignment
~30 min
MS-PS2-2 Homework: Planning and Analyzing Force and Motion Tests
Directions: All data are invented for practice, every force acts along one line, and all motion is measured relative to the ground unless the problem says otherwise. Show the sum of the forces with + and - signs and give units with every number.
Part 1: Sums of Forces and Frames of Reference (Problems 1-2)
Two people paddle a canoe forward with forces of 60 N and 45 N. The water pushes back on the canoe with a drag force of 80 N. (a) Find the sum of the forces. (b) Is the canoe speeding up, slowing down or keeping a steady speed? (c) Later, the drag grows to 105 N while both keep paddling the same. Describe the motion now. (d) Then the second paddler stops while the drag is still 105 N. Find the new sum and describe the change in motion.
A train moves along a straight, level track at a steady 20 m/s. A suitcase sits on the floor of the train. (a) What is the suitcase's speed relative to the train, and relative to a person standing on the platform? (b) What is the sum of the forces on the suitcase? Explain using balanced forces. (c) A passenger walks toward the front of the train at 1.5 m/s relative to the train. What is her speed relative to the platform? (d) A scientist writes a report about the passenger's motion. What must the report state, besides the speed and its units?
Part 2: Analyzing Data (Problems 3-4)
A class uses a simulation with friction turned off. The sum of the forces on a crate is always 80 N for 3.0 s, starting from rest, and only the mass changes. Masses 40, 80, 120 and 160 kg give speeds of 6.0, 3.0, 2.0 and 1.5 m/s. (a) Describe the pattern. (b) Compare the results for 40 kg and 80 kg, and for 80 kg and 160 kg. What happens each time the mass doubles? (c) Predict the speed for a 240 kg crate. (d) Name two variables the class kept the same.
A 1.0 kg drone has a weight of 9.8 N. Its flight log (invented) shows the upward push of its propellers, called lift, and how its motion changed: lift 9.8 N, no change in speed; lift 11.8 N, speed up 2.0 m/s each second upward; lift 13.8 N, speed up 4.0 m/s each second upward; lift 7.8 N, speed up 2.0 m/s each second downward. (a) Find the sum of the forces for each row (up is +). (b) Describe the pattern between the sum and the change in speed. (c) Predict the lift needed to speed up 3.0 m/s each second upward. (d) With a lift of 9.8 N, could the drone be rising? Explain.
Part 3: Plan and Critique (Problems 5-6)
Plan an investigation to test whether the number of identical battery fans on a small foam boat changes how quickly the boat speeds up across a long tub of water. Write (a) the question, (b) the independent variable, (c) the dependent variable and its units, (d) three controlled variables, (e) the number of trials, and (f) a blank data table with headings and units.
A student's plan says: "I will kick a tennis ball, a soccer ball and a basketball across the gym and see which one speeds up the most." (a) List three problems with this plan. (b) Rewrite it as a plan that tests only the effect of mass. (c) Explain how your plan keeps the sum of the forces the same in every trial.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Sum of Forces
Forces combined with correct signs and units; balanced or unbalanced correctly stated
Sum correct in size but direction or units missing
Using Data
Patterns for force and mass described with numbers from the data; predictions correct
Pattern described in words only, or one prediction wrong
Planning
One independent variable, a measured dependent variable with units, three controlled variables, repeated trials
Plan changes one variable but misses units, controls or trials
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data are invented, every force acts along one line, and motion is measured relative to the ground unless a question says otherwise. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
A box is pushed to the right with 25 N. Friction pulls on it with 10 N to the left, and a second person pushes it with 5 N to the left. What is the sum of the forces?
Answer: B
Call right +: 25 - 10 - 5 = +10 N, so 10 N to the right. Choice A adds all three numbers without signs. Choice C forgets the second person's 5 N push. Choice D has the right size but the wrong direction.
Question 2 of 20 · Multiple Choice
The forces on an object are balanced. Which statement could be true?
Answer: C
Balanced forces mean no change in motion, so an object can be at rest or moving at a steady speed in a straight line (Newton's first law). Choice A mixes up "no change in motion" with "no motion." Choices B and D describe changes in motion, which need unbalanced forces.
Question 3 of 20 · Multiple Choice
The same sum of forces acts for 1 s on a 2 kg ball and on a 6 kg ball, both starting at rest. The 2 kg ball gains 3 m/s. How much speed does the 6 kg ball gain?
Answer: D
The 6 kg ball has 3 times the mass, so the same sum of forces gives it one third of the change: 3 ÷ 3 = 1 m/s. Choice B multiplies by 3 instead of dividing. Choice C ignores the mass. Choice A divides the 3 m/s by the 6 kg mass, which mixes up the units.
Question 4 of 20 · Multiple Choice
A student tests how mass affects the speed-up of a skateboard pulled by stretched rubber bands. Which must stay the same in every trial?
Answer: A
To test mass, the pull (the force from the bands) must be controlled. Choices B and D are the mass, which is the independent variable here. Choice C is the dependent variable, the result being measured.
Question 5 of 20 · Multiple Choice
A plan says: "Trial 1: one rubber band on a 0.5 kg puck. Trial 2: two rubber bands on a 1.0 kg puck. Trial 3: three rubber bands on a 1.5 kg puck." What is the main problem?
Answer: C
Both the force and the mass change from trial to trial, so any difference in motion could come from either one. The plan should change only one of them. Choice A is a fair suggestion, but adding trials would not fix the two-variable problem.
Question 6 of 20 · Multiple Choice
Which pair of units should a student use for the sum of the forces and the mass in a data table?
Answer: A
Force is measured in newtons and mass in kilograms. Choice B swaps them. Choice C gives a unit of speed for the force, and choice D gives a unit of length for the mass.
Question 7 of 20 · Multiple Choice
A roller skate carrying a 2.0 kg backpack is pulled by 1, 2 and 3 identical rubber bands, each stretched to the same mark. In the first 0.5 s it travels 0.11, 0.19 and 0.31 m. Which conclusion do the data support?
Answer: D
Two bands give about 1.7 times the distance of one band, and three bands about 2.8 times, close to 2 and 3 for real measurements. So a larger sum of forces gives a larger change in motion. Choices A and B contradict the data, and choice C cannot be tested here, because the mass did not change.
Question 8 of 20 · Multiple Choice
A student sits on a bus moving at a steady 10 m/s and holds a phone still in her lap. What is the phone's speed relative to the bus, and relative to the road?
Answer: B
The phone moves with the bus, so relative to the bus it is at rest (0 m/s), and relative to the road it moves at the bus's 10 m/s. Choice A swaps the two frames. Choices C and D give the same speed in both frames, which ignores the frame of reference.
Question 9 of 20 · Multiple Choice
In a tug-of-war, Team A pulls the rope to the left with 400 N and Team B pulls it to the right with 380 N. What is the sum of the forces on the rope?
Answer: C
Opposite forces subtract: 400 - 380 = 20 N, in the direction of the larger pull, to the left. Choice A adds forces that point in opposite directions. Choice B has the wrong direction. Choice D treats nearly equal forces as balanced, but a 20 N difference is unbalanced.
Question 10 of 20 · Multiple Choice
A 50 kg crate hangs at rest from a crane cable. Its weight is 490 N. The operator increases the cable's pull to 540 N. What is the sum of the forces on the crate now?
Answer: D
Call up +: 540 - 490 = +50 N, so 50 N up, and the crate starts to rise. Choice A adds the weight instead of subtracting it. Choice B forgets the weight. Choice C has the right size but the wrong direction.
Question 11 of 20 · Multiple Choice
Which question can be answered with one investigation that changes only one variable?
Answer: B
Choice B changes only the mass and keeps the sum of the forces fixed, so any change in motion can be traced to the mass. Choice A asks about two variables at once, choice D about three, and choice C is not something a measurement can answer.
Question 12 of 20 · Multiple Choice
A class graphs the distance a box travels in 2.0 s against the spring scale's pull. The scale was zeroed and checked. The line crosses the pull axis at 2.0 N instead of at 0 N. What is the best explanation?
Answer: C
Until the pull is larger than the friction, the forces are balanced and the box does not speed up, so the first 2.0 N of pull only cancels the 2.0 N of friction. Choice A confuses mass with force. Choice D would make the box move with less pull, not more.
Question 13 of 20 · Multiple Choice
An air hockey puck at rest is pushed with a steady 0.018 N for 1.0 s and gains 0.60 m/s. If the same puck is pushed with a steady 0.054 N for 1.0 s, how much speed does it gain?
Answer: A
The force is 3 times as large and the mass is the same, so the change in speed is 3 times as large: 3 × 0.60 = 1.8 m/s. Choice B divides by 3 instead of multiplying. Choice C ignores the larger force. Choice D doubles the speed, as if the force had doubled instead of tripled.
Question 14 of 20 · Multiple Choice
A curling stone slides across the ice at a nearly steady speed. What is the sum of the forces on it?
Answer: D
A nearly steady speed means the motion is hardly changing, so the forces are nearly balanced and the sum is close to zero; the tiny friction from the ice slowly slows it. Choice A reflects the idea that motion needs a forward force. Choice B confuses one force (the weight, balanced by the ice pushing up) with the sum.
Question 15 of 20 · Short Answer
Three forces act on a box along a line: 18 N east, 12 N east and 25 N west. Find the sum of the forces. If the box starts at rest, how does its motion change?
Call east +: 18 + 12 - 25 = 5 N east. The forces are unbalanced, so the box starts moving and speeds up toward the east.
Question 16 of 20 · Short Answer
A 60 kg sled and a 90 kg sled (each with its rider) are each pulled from rest with the same sum of forces, 30 N, for 3 s. The 60 kg sled gains 1.5 m/s. How much speed does the 90 kg sled gain? Explain.
The 90 kg sled has 1.5 times the mass, so the same sum of forces gives it a smaller change in speed: 1.5 ÷ 1.5 = 1.0 m/s. Mass resists changes in motion.
Question 17 of 20 · Short Answer
Write a plan to test whether the sum of the forces on a roller skate changes how much it speeds up. Name the independent variable, the dependent variable with units, two controlled variables and the number of trials.
Sample plan: pull the skate with 1, 2, 3 and 4 identical rubber bands, each stretched to the same mark. Independent variable: the sum of the forces (number of bands, or the pull in N). Dependent variable: distance traveled in the first 0.5 s from rest, in meters. Controlled: the skate's load (mass), the floor surface, the stretch of each band, the start from rest. Trials: at least three for each number of bands, measured relative to the floor.
Question 18 of 20 · Short Answer
A student pulls a box at a steady speed with a spring scale that reads 4.5 N. What is the friction force on the box, and what is the sum of the forces? Explain.
Friction is 4.5 N, opposite to the pull, and the sum of the forces is 0 N. The speed is steady, so the motion is not changing, which means the forces are balanced.
Question 19 of 20 · Short Answer
A sled glides past two students. One, standing on the ground, measures its speed as 1.2 m/s. The other, walking beside the sled at 1.2 m/s, says the sled's speed is 0 m/s. Who is right, and how should the class report the speed?
Both are right in their own frames of reference: the sled moves at 1.2 m/s relative to the ground and at 0 m/s relative to the walking student. The class should report 1.2 m/s relative to the ground and always state the frame, since the ground is the frame used for the whole investigation.
Question 20 of 20 · Short Answer
A class pulls objects with the same sum of forces for 2.0 s, starting from rest. Masses of 1.0, 2.0 and 4.0 kg gain speeds of 1.6, 0.8 and 0.4 m/s. Describe the pattern and predict the speed gained by an 8.0 kg object.
Each time the mass doubles, the speed gained is cut in half: a larger mass gives a smaller change in motion for the same sum of forces. For 8.0 kg (double 4.0 kg), the speed gained is about 0.4 ÷ 2 = 0.2 m/s.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does MS-PS2-2 mean?
MS-PS2-2 asks middle school students to plan an investigation that shows an object's change in motion depends on the sum of the forces on it and on its mass. The focus is on planning a fair test, not on formulas: students choose what to change, what to measure and what to keep the same. The code means Middle School, Physical Science, core idea 2 (Motion and Stability: Forces and Interactions), performance expectation 2.
What grade level is MS-PS2-2?
It is a grades 6-8 standard. Many schools teach it in grade 7 or grade 8 physical science, in the same unit as MS-PS2-1 on Newton's third law. Students need positive and negative numbers to add forces in opposite directions.
What is the assessment boundary for MS-PS2-2?
The official Assessment Boundary limits assessment to forces and motion in one dimension, in an inertial reference frame, and to changing one variable at a time; trigonometry is not included. In practice, that means forces along one line, motion measured from the ground or a steady-moving frame, and plans that change either the force or the mass. Forces at an angle, which need trigonometry, are beyond this standard.
What is the difference between balanced and unbalanced forces?
Balanced forces add to zero, so the object's motion does not change. Unbalanced forces have a sum that is not zero, so the object speeds up, slows down or changes direction. A book resting on a table and a car moving at a steady speed on a straight road both have balanced forces.
Does MS-PS2-2 include F = ma?
Not as a formula. The Clarification Statement asks for qualitative comparisons of forces, mass and changes in motion, so students say "a larger sum gives a larger change" and "more mass gives a smaller change," often with simple ratios. Calculating acceleration with F = ma comes in high school, in HS-PS2-1.
What is a frame of reference in physical science?
A frame of reference is the place or object you measure motion from, such as the ground, a moving bus or a moving walkway. The same object can have different speeds in different frames, so a report should always state the frame. Middle school investigations usually use the ground or the classroom floor.
Why should students change only one variable at a time?
If two things change at once, such as the pull and the mass, there is no way to tell which one caused the result. Changing one variable while keeping the others the same lets students link the result to that one cause. The Assessment Boundary for MS-PS2-2 limits assessment to this kind of plan.
What equipment do students need for force and motion investigations?
A spring scale, something that slides or rolls (a box on furniture sliders, a roller skate, a puck), masses such as water bottles, a meter stick, tape and a stopwatch or phone video are enough. Free online forces-and-motion simulations are useful when there is no space, or to remove friction.
Why does friction matter in these labs?
Friction is one of the forces in the sum. If students treat the pull alone as the sum of the forces, they will think the force changed when only the mass did, because a heavier box has more friction. Measuring friction with a steady-speed pull, and including it in the sum, fixes both problems.
How does MS-PS2-2 connect to high school physics?
It prepares students for HS-PS2-1, where they analyze data to support Newton's second law, Fnet = ma, using the same ideas with full calculations. The planning skills also support MS-PS3-1, where students graph how kinetic energy depends on mass and speed.
07
Related Standards
5 standards
These standards connect to MS-PS2-2: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
3-PS2-1Prerequisite
Investigate how balanced and unbalanced forces affect an object's motion
Lesson coming soon
Alongside
MS-PS2-1Parallel
Apply Newton's third law to design a solution for two colliding objects
NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.
