Force and Motion

Force and Motion

Middle School

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Learning Objectives

Key Vocabulary

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Force

A push or pull that can cause an object to change its motion or direction.

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Motion

The change in an object's position with respect to time and other reference points.

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Speed

The rate of change in an object's position or the distance traveled over time.

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Velocity

The measure of an object's speed combined with its specific direction of motion.

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Acceleration

The rate of change in velocity, such as speeding up, slowing down, or changing direction.

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Net Force

The sum of all forces acting on a body which determines its overall acceleration.

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What are Force and Motion?

• A force is a push or pull that can cause an object to move.

• Motion is a change in position compared to a non-moving reference point.

• Force can change stored potential energy into the energy of motion.

• The energy of motion is also known as kinetic energy.

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Multiple Choice

What happens when a force is applied to an object?

• Its potential energy is converted into kinetic energy.

• Its kinetic energy is converted into potential energy.

• Its energy is lost completely.

• Its energy does not change.

Speed, Velocity, and Acceleration

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Speed

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Velocity

• ​Describes an object's speed with a specific direction of motion.

• ​​An example is a car traveling 30 miles per hour north.

• ​Changing speed or direction changes the object's velocity.

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Acceleration

• ​Is defined as any change in the velocity of an object.

• ​​An object accelerates when it is speeding up or slowing down.

• ​It also accelerates if the object is changing its direction.

Multiple Choice

What is the main difference between an object's speed and its velocity?

• Velocity includes direction, while speed does not.

• Speed measures a change in position, while velocity does not.

• Speed is calculated using distance, while velocity is not.

• Velocity is always a larger value than speed.

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Newton's Second Law of Motion

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Multiple Choice

According to the relationship described by $$F=ma$$ , what happens if the net force on an object is increased?

• The object's acceleration will increase.

• The object's mass will increase.

• The object's acceleration will decrease.

• The object's acceleration will not change.

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Balanced and Unbalanced Forces

Balanced Forces

• When balanced forces act on an object, the net force is zero.

• The forces are equal in size and act in opposite directions.

• There is no change in motion, meaning the object has zero acceleration.

Unbalanced Forces

• When unbalanced forces act on an object, the net force is not zero.

• These forces cause a change in the object's motion, called acceleration.

• The net force is found by combining all individual forces acting on it.

Multiple Choice

What is the key difference in the net force between balanced and unbalanced forces?

• Balanced forces result in a net force of zero, while unbalanced forces result in a non-zero net force.

• Balanced forces are always in the same direction, while unbalanced forces are in opposite directions.

• Balanced forces only apply to objects at rest, while unbalanced forces apply to objects in motion.

• Balanced forces cause an object to accelerate, while unbalanced forces do not.

Effects of Forces on Moving Objects

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Zero Net Force

• ​When the net force on an object is zero, it does not accelerate.

• ​​Its velocity stays constant, so it does not change its speed or direction.

• ​The object keeps moving at a steady pace and in a straight line.

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Force in Same Direction

• ​A net force acts on the object in its direction of movement.

• ​​This unbalanced force causes the moving object to accelerate and speed up.

• ​The object’s velocity increases as long as this net force is applied.

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Force in Opposite Direction

• ​A net force pushes against the object’s direction of current motion.

• ​​This causes the object to decelerate, which means that it slows down.

• ​Its velocity decreases, and it may eventually come to a complete stop.

Multiple Choice

What happens to a moving object if the net force acting on it is zero?

• Its velocity remains constant.

• It immediately stops moving.

• It begins to slow down.

• It begins to speed up.

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What is a Frame of Reference?

• To describe motion, you need a system called a frame of reference.

• Measurements are taken from a specific point called a reference point.

• An object’s motion can look different depending on the observer’s viewpoint.

• A person on a train sees themselves as still, unlike an outside observer.

Multiple Choice

Why might a person on a train and a person on the ground describe the train's motion differently?

• Motion is relative and depends on the observer's frame of reference.

• Objects in motion always appear to be moving backward.

• Motion can only be accurately measured from a stationary point.

• Only one observer can be correct about an object's motion.

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Common Misconceptions

Misconception	Correction
Speed and velocity are the same thing.	Velocity includes both speed and direction.
An object with zero net force is always at rest.	It can also move at a constant velocity.
A force is always needed to keep an object moving.	A force is only needed to change an object's motion.

Multiple Select

A car is traveling north at 50 mph. It then turns east while maintaining the same speed. Which of the following has changed?

• Its velocity

• Its speed

• Its mass

• Its acceleration

Multiple Choice

How does applying an unbalanced force in the same direction of an object's movement affect it?

• It will cause the object to speed up.

• It will cause the object to slow down.

• It will have no effect on the object's motion.

• It will change the object's direction.

Multiple Choice

If you apply the same force to a bowling ball and a tennis ball, why does the tennis ball have a greater acceleration?

• Because the tennis ball has less mass.

• Because the tennis ball has more mass.

• Because the force of gravity is stronger on the tennis ball.

• Because the bowling ball has a smoother surface.

Multiple Choice

A book is sliding across a table and eventually slows down and stops. What can you conclude about the forces acting on the book as it slows?

• There is an unbalanced force (friction) acting opposite to the direction of motion.

• The forces on the book are balanced.

• No forces are acting on the book once it starts sliding.

• The book ran out of the force that was pushing it.

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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

• 2

• 3

• 4