Forces and Motion

Forces and Motion

Middle School

​

Learning Objectives

Key Vocabulary

​

Force

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

​

Motion

Motion is the action of changing position, measured in relation to a specific reference point.

​

Speed

Speed is the rate at which an object covers a certain distance over a period of time.

​

Velocity

Velocity is the rate an object changes position, including its specific direction of motion.

​

Acceleration

Acceleration is the rate of change of an object's velocity, including changes in speed or direction.

​

Net Force

Net Force is the overall force acting on an object when all individual forces are combined.

Key Vocabulary

​

Reference Frame

A system or a set of coordinates used to describe the motion of an object accurately.

​

​

Forces, Motion, and Energy

• A force is simply a push or a pull on an object.

• Motion is an object's change in position from a reference point.

• Potential energy is stored energy; kinetic energy is the energy of motion.

• A force can change potential energy to kinetic energy, causing movement.

​

Multiple Choice

Which of the following best defines a force?

• The energy an object has because of its movement.

• The stored energy within an object at rest.

• A push or a pull on an object.

• An object's change in position from a reference point.

Multiple Choice

What is the relationship between a force and an object's energy?

• A force creates motion without affecting energy.

• A force converts an object's kinetic energy to potential energy.

• A force can convert an object's potential energy to kinetic energy.

• A force increases an object's potential energy.

Multiple Choice

A rock is sitting at the top of a hill. Which statement best explains the sequence of events that must occur for it to start moving down the hill?

• The rock must change its position relative to a reference point, which gives it kinetic energy.

• The rock's stored energy must be measured first, which then causes it to move.

• A force, like a small push, must be applied to convert the rock's stored potential energy into kinetic energy.

• The rock's kinetic energy must be converted to potential energy to cause motion.

Speed, Velocity, and Acceleration

​

Speed

​

Velocity

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

• ​​It includes both the speed and the direction of an object.

• ​It can be negative if the object changes its original direction.

​

Acceleration

• ​Acceleration is any change that is made to an object's velocity.

• ​​An object accelerates if it speeds up or it slows down.

• ​Changing direction is also a form of acceleration for any object.

Multiple Choice

What key piece of information is included in an object's velocity but not in its speed?

• How fast the object is moving.

• The distance the object has traveled.

• The time it took for the object to move.

• The direction the object is moving.

Multiple Choice

Which of the following scenarios is the best example of an object accelerating?

• A satellite that has stopped moving in space.

• A student sitting perfectly still in a chair.

• A car driving in a circle at a constant speed.

• A train moving at a constant speed on a straight track.

Multiple Choice

A car starts from a stop, speeds up to 60 mph, travels in a straight line for a mile, then slows down and turns a corner. During which part of this journey is the car accelerating?

• Only when it is traveling in a straight line at 60 mph.

• When it speeds up, slows down, and turns the corner.

• Only when it is turning the corner.

• Only when it is speeding up from a stop.

​

​

Newton's Second Law of Motion

Multiple Choice

Which of the following best describes Newton's Second Law of Motion?

• An object's mass and acceleration are not related to force.

• An object's acceleration depends only on its mass.

• Net force is the same as an object's weight.

• Net force is calculated by multiplying an object's mass by its acceleration.

Multiple Choice

According to the relationship $$F=ma$$ , what is the effect of applying a greater net force to an object?

• The object's mass will increase.

• The object's acceleration will not change.

• The object's acceleration will decrease.

• The object's acceleration will increase.

Multiple Choice

If you push a bowling ball and a tennis ball with the exact same amount of force, why does the tennis ball have a greater acceleration?

• Because the tennis ball is smaller in size.

• Because the tennis ball has less mass.

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

• Because the bowling ball has a smoother surface.

​

​

Balanced and Unbalanced Forces

Balanced Forces

• ​Forces are equal in size and opposite in direction, resulting in a net force of 0 N.

• ​​This means there is no change in the object's motion.

• ​The object will either remain stationary or continue moving at a constant velocity.

Unbalanced Forces

• ​Forces are unequal in size or direction, resulting in a net force greater than 0 N.

• ​​This causes a change in the object’s motion, known as acceleration.

• ​The object will speed up, slow down, or change its direction of motion.

Multiple Choice

What is the result of balanced forces acting on an object?

• The object slows down and stops

• The object accelerates by speeding up

• There is no change in the object's motion

• The object changes its direction

Multiple Choice

What is the relationship between an unbalanced force and an object's motion?

• An unbalanced force causes a change in an object's motion, known as acceleration.

• An unbalanced force causes an object to move at a constant velocity.

• An unbalanced force results in a net force of 0 N, stopping the object.

• An unbalanced force has no effect on an object's motion.

Multiple Choice

A satellite is moving through space at a constant velocity. If it encounters no forces, what will happen to its motion?

• It will change its direction.

• It will continue moving at a constant velocity.

• It will start moving faster.

• It will gradually slow down and stop.

Effects of Forces on Moving Objects

​

Zero Net Force

• When all forces on an object cancel out, the net force is zero.

• A moving object with zero net force maintains a constant velocity.

• This means its speed and direction of motion do not change.

​

Force in Same Direction

• An unbalanced force is applied in the same direction the object is moving.

• This net force causes the object to accelerate and speed up.

• The object's velocity increases as long as the force is applied.

​

Force in Opposite Direction

• An unbalanced force is applied opposite to the object's direction of motion.

• This opposing force, like friction, causes the object to slow down.

• The object's velocity decreases, and it may eventually stop.

Multiple Choice

When a moving object experiences a net force of zero, what happens to its velocity?

• It changes direction.

• It remains constant.

• It increases.

• It decreases.

Multiple Choice

How does applying an unbalanced force in the same direction an object is moving affect its motion?

• It causes the object to speed up.

• It causes the object to maintain a constant velocity.

• It causes the object's direction to change.

• It causes the object to slow down.

Multiple Choice

A toy car is rolling forward. If you give it a push in the same direction it is moving and then it encounters a patch of friction, what is the sequence of changes to its velocity?

• Its velocity first decreases, then increases.

• Its velocity remains constant, then decreases.

• Its velocity first increases, then decreases.

• Its velocity increases, then remains constant.

​

​

What Is a Frame of Reference?

• To describe motion accurately, you need a specific frame of reference.

• Measurements are made from a reference point we treat as stationary.

• An object's motion can appear different from another observer's viewpoint.

• A train seems to move from the ground, but not from inside it.

​

Multiple Choice

What is the main purpose of using a frame of reference?

• To change the direction of a moving object

• To measure the distance between two objects

• To provide a stationary point for measuring motion

• To increase the speed of an object

Multiple Choice

Why might two people describe the motion of the same object differently?

• Because they are in different frames of reference

• Because only one person can measure motion accurately

• Because the object is changing its mass

• Because the object is making noise

Multiple Choice

A passenger is on a bus that is moving forward. If they look at the person sitting in the seat next to them, how would they describe that person's motion?

• The person is moving backward

• The person is not moving

• The person is moving forward

• The person is moving sideways

​

Common Misconceptions

Misconception	Correction
An object in motion must have a force acting on it.	A force is needed to change motion, not to maintain it.
Acceleration always means speeding up.	Acceleration is any change in velocity, including slowing down or changing direction.
If an object is not moving, there are no forces acting on it.	A stationary object can have balanced forces, resulting in zero net force.

​

Summary

Poll

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

• 1

• 2

• 3

• 4