physical science Lesson
Why Don't the Dishes Move? Exploring Newton's Three Laws of Motion
Shared by A STEM Educator
8th GradeNGSS Phenomenon-BasedMS-PS2-250 min
Learning Objective
Students will be able to predict and explain the motion of objects by constructing free-body diagrams, calculating net force and acceleration using F=ma, and applying Newton's Three Laws of Motion to real-world scenarios (MS-PS2-2).
Lesson Overview
Students anchor their learning in the tablecloth trick phenomenon, then build from identifying everyday forces to drawing free-body diagrams, calculating net force and acceleration, and applying all three of Newton's Laws to explain multi-step real-world motion scenarios.
Materials
- Smooth tablecloth and plastic dishes (teacher demo)
- Student investigation handout (printed)
- Rulers and pencils for free-body diagram drawing
- Calculators
- Whiteboard markers and mini whiteboards (or scratch paper)
- Projector for video still / phenomenon image
- Sticky notes for exit slip
Scaffolded Task Progression
1Task 1
IDENTIFYING FORCES IN EVERYDAY SCENARIOS
Look at the four scenarios below. For each one, list ALL the forces acting on the object in bold and identify each force type (gravity, normal, friction, applied, air resistance, tension).
a) A soccer ball rolling across a grass field and slowing down.b) A backpack hanging from a hook on a wall.c) A person pushing a shopping cart forward.d) The tablecloth being yanked off the table. [ Interactive graph / diagram, available in the full lesson ]
a) A soccer ball rolling across a grass field and slowing down.b) A backpack hanging from a hook on a wall.c) A person pushing a shopping cart forward.d) The tablecloth being yanked off the table. [ Interactive graph / diagram, available in the full lesson ]
2Task 2
NEWTON'S FIRST LAW — THE LAW OF INERTIA
Newton's 1st Law states: An object at rest stays at rest, and an object in motion stays in motion at constant speed and direction, UNLESS acted on by a net (unbalanced) force.
Answer the following:
a) A hockey puck slides on ice and gradually slows to a stop. Is this consistent with Newton's 1st Law? What force is responsible for the change?b) An astronaut in deep space (no gravity, no air) gives a space probe a single push. What happens to the probe after the push? Why?c) Return to the tablecloth trick: The dishes are sitting still before the trick. According to Newton's 1st Law, what would need to happen for the dishes to move? Why do they barely move when the cloth is yanked quickly?
a) A hockey puck slides on ice and gradually slows to a stop. Is this consistent with Newton's 1st Law? What force is responsible for the change?b) An astronaut in deep space (no gravity, no air) gives a space probe a single push. What happens to the probe after the push? Why?c) Return to the tablecloth trick: The dishes are sitting still before the trick. According to Newton's 1st Law, what would need to happen for the dishes to move? Why do they barely move when the cloth is yanked quickly?
3Task 3
NEWTON'S THIRD LAW — ACTION-REACTION PAIRS
Newton's 3rd Law states: For every action force, there is an equal and opposite reaction force acting on a DIFFERENT object.
For each situation, identify the action force AND the reaction force. State the object each force acts ON.
a) A swimmer pushes backward on the water with her feet.b) A rocket expels hot gases downward.c) You push down on the ground when you jump.d) The tablecloth is pulled to the right by the performer's hands. ⚠️ CHALLENGE: A student says, 'If action and reaction forces are equal and opposite, they should cancel out and nothing should ever move!' What is wrong with this reasoning?
a) A swimmer pushes backward on the water with her feet.b) A rocket expels hot gases downward.c) You push down on the ground when you jump.d) The tablecloth is pulled to the right by the performer's hands. ⚠️ CHALLENGE: A student says, 'If action and reaction forces are equal and opposite, they should cancel out and nothing should ever move!' What is wrong with this reasoning?
4Task 4
FREE-BODY DIAGRAM DRAWING TASK
A free-body diagram (FBD) shows ALL forces on ONE object as arrows. Arrow length = relative force size. Arrow direction = force direction. Label each arrow with force name and value if known.
Draw a complete, labeled free-body diagram for EACH scenario below. Use the blank boxes provided.
a) A 5 kg book sitting still on a table. (gravity = 49 N down)b) A 10 kg box being pushed to the right with 40 N applied force; friction = 15 N opposing motion; weight = 98 N; normal force = 98 N.c) A 2 kg ball in free fall (only gravity acts; weight = 19.6 N). [ Interactive graph / diagram, available in the full lesson ]
a) A 5 kg book sitting still on a table. (gravity = 49 N down)b) A 10 kg box being pushed to the right with 40 N applied force; friction = 15 N opposing motion; weight = 98 N; normal force = 98 N.c) A 2 kg ball in free fall (only gravity acts; weight = 19.6 N). [ Interactive graph / diagram, available in the full lesson ]
5Task 5
CALCULATING NET FORCE AND PREDICTING MOTION DIRECTION
Using the free-body diagrams you drew in Task 4, answer the following:
a) For the book (Task 4a): Is the net force zero or nonzero? Will the book accelerate? Explain using Newton's 1st Law.b) For the pushed box (Task 4b): What is the net force? In which direction will the box accelerate?c) A new scenario: A tug-of-war rope is pulled by Team A with 350 N to the left and Team B with 410 N to the right. - Draw a quick FBD of the rope. - Calculate the net force. - Which team is winning? How do you know?d) What net force would be needed to keep the rope completely still (in equilibrium)?
a) For the book (Task 4a): Is the net force zero or nonzero? Will the book accelerate? Explain using Newton's 1st Law.b) For the pushed box (Task 4b): What is the net force? In which direction will the box accelerate?c) A new scenario: A tug-of-war rope is pulled by Team A with 350 N to the left and Team B with 410 N to the right. - Draw a quick FBD of the rope. - Calculate the net force. - Which team is winning? How do you know?d) What net force would be needed to keep the rope completely still (in equilibrium)?
6Task 6
DATA TABLE — APPLYING NEWTON'S 2ND LAW (F = ma)
Newton's 2nd Law: Net Force = mass × acceleration → F = ma → a = F/m
Complete the data table below. Calculate the missing value in each row. Round to the nearest tenth. Then answer the analysis questions.
Analysis Questions:
a) Looking at Trials 1 and 4, if net force increases but mass stays the same, what happens to acceleration?b) If you doubled the mass of the object in Trial 1 but kept the force the same, what would the new acceleration be?c) Which trial has the greatest acceleration? Which has the least? What pattern do you notice? [ Interactive graph / diagram, available in the full lesson ]
Newton's 2nd Law: Net Force = mass × acceleration → F = ma → a = F/m
Complete the data table below. Calculate the missing value in each row. Round to the nearest tenth. Then answer the analysis questions.
| Trial | Net Force (N) | Mass (kg) | Acceleration (m/s²) |
|---|---|---|---|
| 1 | 50 | 10 | ? |
| 2 | 120 | ? | 6.0 |
| 3 | ? | 8 | 4.5 |
| 4 | 200 | 25 | ? |
| 5 | 75 | ? | 2.5 |
Analysis Questions:
a) Looking at Trials 1 and 4, if net force increases but mass stays the same, what happens to acceleration?b) If you doubled the mass of the object in Trial 1 but kept the force the same, what would the new acceleration be?c) Which trial has the greatest acceleration? Which has the least? What pattern do you notice? [ Interactive graph / diagram, available in the full lesson ]
7Task 7
MULTI-STEP SCENARIO — APPLYING ALL THREE LAWS: THE SKATEBOARD COLLISION
Jordan (mass = 50 kg) stands on a skateboard at rest. Jordan pushes off a wall with a force of 30 N. The skateboard + Jordan system has a total mass of 55 kg.
Stage 1 — The Push:
a) Draw a free-body diagram of Jordan + skateboard at the moment of the push. Label all forces.b) Identify the action force and reaction force. Which law applies?c) Calculate Jordan's acceleration during the push. Stage 2 — Rolling:d) After the push, friction from the ground = 5 N opposing motion. What is the net force now? e) Will Jordan speed up, slow down, or move at constant speed? Which law explains this? Stage 3 — Stopping: f) Jordan grabs a fence post and stops in 0.5 seconds. If the stopping force is 110 N, what was Jordan's deceleration? (Use a = F/m) [ Interactive graph / diagram, available in the full lesson ]
a) Draw a free-body diagram of Jordan + skateboard at the moment of the push. Label all forces.b) Identify the action force and reaction force. Which law applies?c) Calculate Jordan's acceleration during the push. Stage 2 — Rolling:d) After the push, friction from the ground = 5 N opposing motion. What is the net force now? e) Will Jordan speed up, slow down, or move at constant speed? Which law explains this? Stage 3 — Stopping: f) Jordan grabs a fence post and stops in 0.5 seconds. If the stopping force is 110 N, what was Jordan's deceleration? (Use a = F/m) [ Interactive graph / diagram, available in the full lesson ]
8Task 8
HIGH-CEILING — CAR ON A HILL: ALL THREE LAWS IN A MULTI-STEP SCENARIO
A 1,200 kg car is driving up a hill. Analyze each stage using Newton's Laws. Show all work including FBDs and calculations.
Stage 1 — Accelerating from rest at the bottom of the hill:
- Engine force forward = 4,800 N
- Friction + air resistance = 1,200 N
- The hill adds a gravity component opposing motion = 1,400 N
a) Draw an FBD for the car at this stage.b) Calculate the net force.c) Calculate the car's acceleration.d) Which of Newton's Laws explains why the car accelerates in the direction of net force? Stage 2 — Cruising at constant speed up the hill: e) What must be true about the net force? What does Newton's 1st Law tell us? f) If friction + air resistance + gravity component still = 2,600 N backward, what must the engine force equal? Stage 3 — Braking hard at the top of the hill: - Brakes apply 5,000 N backward - Engine force = 0 - Friction = 800 N backward - Gravity component now = 600 N FORWARD (car is cresting the hill, gravity pulls it forward/downhill) g) Draw an FBD for Stage 3. h) Calculate net force and acceleration during braking. i) A passenger in the car lurches forward during braking. Which law explains this? Why? j) The car's tires push backward on the road during the drive. What pushes the car forward? Which law is this? [ Interactive graph / diagram, available in the full lesson ]
a) Draw an FBD for the car at this stage.b) Calculate the net force.c) Calculate the car's acceleration.d) Which of Newton's Laws explains why the car accelerates in the direction of net force? Stage 2 — Cruising at constant speed up the hill: e) What must be true about the net force? What does Newton's 1st Law tell us? f) If friction + air resistance + gravity component still = 2,600 N backward, what must the engine force equal? Stage 3 — Braking hard at the top of the hill: - Brakes apply 5,000 N backward - Engine force = 0 - Friction = 800 N backward - Gravity component now = 600 N FORWARD (car is cresting the hill, gravity pulls it forward/downhill) g) Draw an FBD for Stage 3. h) Calculate net force and acceleration during braking. i) A passenger in the car lurches forward during braking. Which law explains this? Why? j) The car's tires push backward on the road during the drive. What pushes the car forward? Which law is this? [ Interactive graph / diagram, available in the full lesson ]
9Task 9
SYNTHESIS — EXPLAINING THE TABLECLOTH TRICK USING ALL THREE LAWS
You have now studied all three of Newton's Laws. Return to the anchor phenomenon: the tablecloth trick.
Write a complete scientific explanation (4–6 sentences minimum) that answers the driving question:
'Why do the dishes stay in place when the tablecloth is pulled away?'
Your explanation MUST:
✅ Reference Newton's 1st Law and the concept of inertia
✅ Reference Newton's 2nd Law — explain why the short contact time matters (think: small force × short time = tiny effect on dish velocity)
✅ Reference Newton's 3rd Law — what force does the cloth exert on the dishes, and what do the dishes exert back?
✅ Use at least THREE of these vocabulary terms: inertia, net force, acceleration, mass, friction, action-reaction
✅ Include a labeled free-body diagram of ONE dish during the trick
Extension: Would heavier dishes make the trick easier or harder to perform? Use F=ma to justify your answer quantitatively.
You have now studied all three of Newton's Laws. Return to the anchor phenomenon: the tablecloth trick.
Write a complete scientific explanation (4–6 sentences minimum) that answers the driving question:
'Why do the dishes stay in place when the tablecloth is pulled away?'
Your explanation MUST:
✅ Reference Newton's 1st Law and the concept of inertia
✅ Reference Newton's 2nd Law — explain why the short contact time matters (think: small force × short time = tiny effect on dish velocity)
✅ Reference Newton's 3rd Law — what force does the cloth exert on the dishes, and what do the dishes exert back?
✅ Use at least THREE of these vocabulary terms: inertia, net force, acceleration, mass, friction, action-reaction
✅ Include a labeled free-body diagram of ONE dish during the trick
Extension: Would heavier dishes make the trick easier or harder to perform? Use F=ma to justify your answer quantitatively.
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