Force and Motion

Force and Motion

Middle School

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Learning Objectives

• Define motion relative to a reference frame.

• Differentiate between contact forces and noncontact forces.

• Explain Newton's three laws of motion and apply them to real-world scenarios.

• Describe the factors that affect friction and gravity.

Key Vocabulary

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Motion

The change in position of an object when compared to a non-moving object called a reference frame.

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Force

A push or a pull on an object, described by its strength in Newtons (N) and its direction.

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Net Force

The combination of all forces acting on an object, determining how the object's motion will change.

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Inertia

The resistance of an object to any change in its state of motion, including its speed and direction.

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Friction

A contact force that two surfaces exert on each other when they rub against each other, opposing motion.

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Gravity

A noncontact force of attraction that pulls objects toward each other as a result of their respective masses.

Key Vocabulary

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Speed

The rate at which an object covers distance in a certain amount of time.

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Velocity

The speed of an object moving in a specific direction, a complete description of motion.

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Acceleration

The rate at which velocity changes, which can be a change in speed or direction.

Describing Motion and Force

• Motion is a change in position when compared to a reference frame.

• Because the reference frame can change, all motion is considered relative.

• A force is a push or a pull, described by strength and direction.

Multiple Choice

What is required to determine that an object is in motion?

• A change in position compared to a reference frame.

• The strength of a push or a pull.

• The speed and direction of an object.

• A change in an object's mass or weight.

Multiple Choice

Why is motion described as being 'relative'?

• Because it is always caused by a push or a pull.

• Because its description depends on the chosen reference frame.

• Because it can only happen when a force is weak.

• Because it is measured by an object's strength and direction.

Multiple Choice

An astronaut in space gives a floating toolbox a push, and it drifts away. How can this event be described using the definitions of force and motion?

• The push is the force, and the change in the toolbox's position is the motion.

• The drifting of the toolbox is the force, and the push is the motion.

• The push is the force, but there is no motion because it's in space.

• The change in position is the force, and the push is the reference frame.

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Types of Forces

Contact Forces

• Contact forces are applied only when one object actually touches another object.

• Friction is a key example of a contact force that you can observe daily.

• This force occurs when two surfaces rub against each other, opposing the motion.

Noncontact Forces

• Noncontact forces are applied to an object whether it touches the object or not.

• Gravity is a primary example, as it pulls objects toward each other based on mass.

• Magnetism and electrical forces are other examples of noncontact forces in action.

Multiple Choice

What is the fundamental difference between contact and noncontact forces?

• Contact forces require objects to touch, while noncontact forces do not.

• Contact forces push objects apart, while noncontact forces pull them together.

• Contact forces are weaker than noncontact forces.

• Contact forces relate to an object's mass, while noncontact forces relate to its motion.

Multiple Choice

How does friction act as a contact force?

• Because it occurs when two surfaces rub against each other, opposing motion.

• Because it pulls objects toward each other based on their mass.

• Because it can be applied to an object without touching it.

• Because it is a force that includes magnetism and electricity.

Multiple Choice

A student pushes a book, and it slides across a flat table before stopping. What combination of forces best explains why the book slows down and stays on the table?

• The contact force of friction and the noncontact force of gravity.

• Only the contact force from the push, which eventually runs out.

• Only the noncontact force of gravity, which slows the book down.

• The noncontact forces of magnetism and electricity from the table.

Balanced and Unbalanced Forces

• Balanced forces are equal and opposite, causing no change in motion.

• Unbalanced forces are unequal, causing a change in an object's motion.

• The net force is the combination of all forces acting on an object.

Multiple Choice

What is the result of an unbalanced force acting on an object?

• It causes a change in the object's motion.

• It has no effect on the object's motion.

• It makes the object's mass smaller.

• It only makes the object stop moving.

Multiple Choice

If an object's motion is not changing, what can be concluded about the forces acting on it?

• The forces are balanced, resulting in a net force of zero.

• The forces are unbalanced, resulting in a net force of zero.

• The object is accelerating very quickly.

• Only one force is acting on the object.

Multiple Choice

Imagine one person pushes a box with a force of 50N to the right, while another person pushes the same box with a force of 30N to the left. What is the most likely outcome?

• The box will move in the direction of the 50N push because the forces are unbalanced.

• The box will not move because the forces are balanced.

• The box will move in the direction of the 30N push because it is a smaller force.

• The box will move back and forth between the two forces.

Speed and Velocity

• Speed is the measure of how fast an object is moving.

• Average speed is total distance over total time for a trip.

• Velocity is the speed of an object in a specific direction.

Multiple Choice

What does the speed of an object measure?

• How fast the object is moving

• The direction the object is traveling

• The object's total mass

• The force applied to the object

Multiple Choice

If a bus travels 200 kilometers in 4 hours, what is its average speed?

• 50 km/h

• 100 km/h

• 200 km/h

• 800 km/h

Multiple Choice

Why can two trains traveling at the same speed have different velocities?

• Because they are moving in different directions

• Because they have different starting points

• Because they travel for different amounts of time

• Because they have different average speeds

Acceleration

• An object accelerates when its speed increases.

• An object also accelerates when its speed decreases.

• Changing direction, even at a constant speed, is also acceleration.

Multiple Choice

What is acceleration?

• A change in an object's speed or direction.

• The speed at which an object is moving.

• An increase in an object's mass.

• The distance an object travels over time.

Multiple Choice

A car is driving in a circle at a steady speed of 30 mph. Why is the car considered to be accelerating?

• Because its speed is increasing.

• Because its direction is constantly changing.

• Because its speed is decreasing.

• Because it is moving faster than a person can walk.

Multiple Choice

A train travels from one city to another. It starts from rest, moves in a straight line at a constant high speed, and then slows down to a stop at the next station. During which parts of the journey is the train accelerating?

• Only when traveling at a constant high speed.

• Only when starting up and slowing down.

• During the entire trip.

• The train is never accelerating.

Newton's First and Second Laws

Multiple Choice

What is the term for an object's resistance to a change in its motion?

• Inertia

• Acceleration

• Force

• Motion

Multiple Choice

Under what condition will an object's motion change?

• An unbalanced force is applied to it.

• Its inertia is increased.

• The object's mass is decreased.

• It is in a state of constant motion.

Multiple Choice

If you apply the same amount of force to a bowling ball and a soccer ball, which will have the greater acceleration?

• The soccer ball, because it has less mass.

• The bowling ball, because it is heavier.

• They will both accelerate at the same rate.

• Neither will accelerate without more force.

Newton's Third Law

• For every action, there is an equal and opposite reaction.

• Action-reaction forces act on two different objects.

• Because they act on different objects, they do not cancel out.

Multiple Choice

Which statement best defines Newton's Third Law?

• For every action, there is an equal and opposite reaction.

• An object in motion stays in motion.

• Force is equal to mass times acceleration.

• Action-reaction forces always cancel each other out.

Multiple Choice

What is the relationship between the two forces in an action-reaction pair?

• They act on two different objects.

• One force is always stronger than the other.

• They only occur on objects that are not moving.

• Gravity interferes with one of the forces.

Multiple Choice

A swimmer pushes backward on the water, and the water pushes the swimmer forward. Which statement provides the best conclusion about the forces in this scenario?

• The forces are an action-reaction pair that act on different objects and do not cancel out.

• The forces cancel each other out because they are equal and opposite.

• The swimmer pushes on the water with more force than the water pushes on the swimmer.

• The water's reaction force is delayed and happens after the swimmer stops pushing.

Friction

• Friction is a force that opposes motion between two surfaces in contact.

• Friction depends on the types of surfaces and how hard they are pushed.

• The four main types of friction are static, sliding, rolling, and fluid.

Multiple Choice

Which statement best defines friction?

• A force that causes objects to start moving

• A force that only exists in liquids and gases

• A force that increases an object's speed

• A force that opposes motion between surfaces in contact

Multiple Choice

What are the two main factors that determine the amount of friction?

• The color of the surfaces and their temperature

• The speed of the object and its mass

• The types of surfaces and how hard they are pushed together

• The size of the object and the time it is in motion

Multiple Choice

A student needs to move a heavy box of books across a floor. Which strategy uses a different type of friction to make the box easiest to move?

• Removing some contents from the box to make it lighter

• Placing the box on a wheeled cart before pushing it

• Pushing the box across a rougher surface like a carpet

• Asking a friend to push down on the box

Universal Gravitation

• Gravity is a force of attraction between any two objects with mass.

• The gravitational force increases as the mass of the objects increases.

• The gravitational force decreases as the distance between objects increases.

Multiple Choice

Which statement best defines gravity?

• A force that pushes two objects apart.

• A force of attraction between any two objects with mass.

• A force that only exists between planets and stars.

• A type of energy that objects store.

Multiple Choice

What happens to the gravitational force between two objects if their masses increase?

• The gravitational force between them increases.

• The gravitational force between them decreases.

• The gravitational force between them is unchanged.

• The gravitational force disappears completely.

Multiple Choice

Imagine two pairs of objects. Pair 1 consists of two objects with large masses that are close together. Pair 2 consists of two objects with small masses that are far apart. Which pair would experience a stronger gravitational force, and why?

• Pair 1, because gravitational force increases with larger masses and decreases with greater distance.

• Pair 1, because gravitational force only depends on distance, not mass.

• Pair 2, because gravitational force increases when objects are far apart.

• Pair 2, because smaller masses always attract each other more strongly.

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Common Misconceptions

Misconception	Correction
Acceleration only means ‘speeding up.’	Acceleration is any change in velocity, including slowing down or changing direction.
Action-reaction force pairs cancel out.	These forces act on different objects, so they do not cancel.
An object in motion must have a force acting on it.	A force is only required to change an object's motion (to accelerate it).
Mass and weight are the same thing.	Mass is matter in an object; weight is the force of gravity.

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Summary

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1 - Not at all confident

• 2 - A little confident

• 3 - Mostly confident

• 4 - Very confident