physical science Lesson

Why Don't the Dishes Move? Exploring Newton's Three Laws of Motion

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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 ]

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?

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?

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