Newton's Laws of Motion

Newton's Laws of Motion

Middle School

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Learning Objectives

Key Vocabulary

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Inertia

The natural tendency of any object is to resist a change in its current state of motion.

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Net Force

The overall force acting on an object when all the individual forces are combined together.

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Newton's First Law

An object stays at rest or in motion unless an external net force is applied to it.

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Newton's Second Law

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Newton's Third Law

This law states that for every action, there is an equal and opposite reaction from another object.

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Newton (N)

The Newton (N) is the standard unit of force, equivalent to 1 kg-m/s².

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Newton's First Law: The Law of Inertia

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Multiple Choice

What is the relationship between an object's mass and its inertia?

• An object with more mass has more inertia.

• An object with more mass has less inertia.

• An object's mass has no effect on its inertia.

• Only objects at rest have inertia.

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Force, Mass, and Acceleration

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Multiple Choice

If a larger net force is applied to an object, what is the resulting effect on its motion?

• The object will have a greater acceleration.

• The object will have a smaller acceleration.

• The object's mass will increase.

• The object's velocity will not change.

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Calculating with Newton's Second Law

Solving for Acceleration

Solving for Mass

Multiple Choice

If a net force of 20 newtons is applied to a 5-kilogram object, what will be the object's acceleration?

• 4 m/s²

• 0.25 m/s²

• 15 m/s²

• 100 m/s²

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Newton's Third Law: Action and Reaction

• For every action, there is an equal and opposite reaction; forces always occur in pairs.

• These pairs are known as action-reaction pairs, with forces of equal strength and opposite direction.

• For example, when you push a wall, the wall pushes back on you.

• These action-reaction forces always act on two different objects.

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Multiple Choice

What are the key characteristics of an action-reaction force pair?

• They are equal in strength and opposite in direction.

• They are equal in strength and in the same direction.

• They are unequal in strength and opposite in direction.

• They act on the same object at the same time.

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Collisions and Newton's Third Law

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Multiple Choice

In a collision between a large truck and a small car, which statement explains why the car experiences a greater acceleration?

• The forces are equal, but the car has less mass.

• The truck exerts a much greater force on the car.

• The road exerts a stronger force on the car.

• The car has more momentum than the truck.

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Action-Reaction vs. Balanced Forces

Balanced Forces

• ​Balanced forces act on a single object and are equal in size but opposite in direction.

• ​​These forces cancel each other out, resulting in a net force of zero.

• ​As a result, there is no change in the object's state of motion.

Action-Reaction Forces

• ​Action-reaction forces are a pair of forces that act on two different objects.

• ​​They are also equal in size and opposite in direction, just like balanced forces.

• ​However, they do not cancel out since they act on two separate objects.

Multiple Choice

What is the key difference used to distinguish between balanced forces and action-reaction forces?

• The object or objects that the forces are acting on.

• The strength or magnitude of the forces.

• The direction in which the forces are pointing.

• The type of friction involved in the forces.

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What Is a Frame of Reference?

• A frame of reference is a viewpoint for observing an object’s motion.

• ​It helps measure an object's position, orientation, and movement over time.

• How motion appears depends on the observer's frame of reference.

• Scientists use a common frame to compare and share motion data accurately.

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Multiple Choice

Why would a person on a moving train appear still to a fellow passenger but seem to be in motion to an observer on a platform?

• The motion is being observed from different frames of reference.

• The train is accelerating too quickly to measure.

• The person on the platform is not moving.

• Only one observer is using scientific units correctly.

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Common Misconceptions

Misconception	Correction
An object at rest has no forces acting on it.	An object at rest has balanced forces, resulting in a zero net force.
Action-reaction forces cancel each other out.	Action-reaction forces act on different objects, so they do not cancel out.
Inertia is a force that keeps things moving.	Inertia is a property of mass that resists a change in an object's motion.
Mass and weight are the same thing.	Mass is the amount of matter in an object, while weight is the force of gravity.

Multiple Choice

A swimmer pushes backward on the water to move forward. Why don't these two forces cancel each other out, according to Newton's laws?

• Because they are an action-reaction pair acting on different objects (the swimmer and the water).

• Because the force from the swimmer is stronger than the force from the water.

• Because the water has more inertia than the swimmer.

• Because they are balanced forces that are not equal in strength.

Multiple Choice

If two objects with different masses collide, which statement accurately describes the forces and acceleration?

• The forces are equal, but the object with less mass has greater acceleration.

• The object with more mass exerts a greater force, causing equal acceleration.

• The forces are equal, and the acceleration of both objects is the same.

• The forces are unequal, but the acceleration is the same for both objects.

Multiple Choice

A rocket accelerates upward by expelling hot gas downward. How do all three of Newton's Laws apply to this situation?

• The rocket stays in motion (1st Law), the force of the gas creates acceleration (2nd Law), and the rocket pushes on the gas while the gas pushes on the rocket (3rd Law).

• The rocket has inertia (1st Law), the mass of the rocket determines the force (2nd Law), and the forces of gravity and thrust are balanced (3rd Law).

• The rocket overcomes gravity (1st Law), the acceleration is constant (2nd Law), and the action-reaction forces are on the rocket itself (3rd Law).

• Gravity is the only force acting on the rocket (1st Law), the rocket's weight is its mass times acceleration (2nd Law), and the upward and downward forces cancel out (3rd Law).

Multiple Choice

An empty shopping cart and a full one are pushed with the same force. What would you predict about their motion and why?

• The empty cart will accelerate more because it has less mass $$a=F/m$$ .

• Both carts will have the same acceleration because the force is the same.

• The full cart will accelerate more because it has more inertia.

• Neither cart will move because the forces are balanced by friction.

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Summary

Poll

On a scale of 1-4, how confident are you about applying Newton's Laws of Motion to solve problems?

• 1

• 2

• 3

• 4