Describing Motion and Force

Describing Motion and Force

Middle School

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Learning Objectives

• Define motion by explaining the importance of a reference frame and appropriate units.

• Explain how the sum of the forces (net force) and an object's mass affect its change in motion.

• Differentiate between balanced forces (Newton's First Law) and unbalanced forces (Newton's Second Law).

• Distinguish between contact forces, like friction, and noncontact forces, like gravity.

Key Vocabulary

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Reference Frame

A reference frame is a place or an object that is used for comparison to describe motion.

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Relative Motion

Relative motion is movement that is described in relation to a specific, often moving, reference frame.

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Force

A force is simply defined as any push or any pull that is exerted on an object.

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Newton (N)

The newton is the standard international (SI) unit used to measure the strength or magnitude of a force.

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Contact Force

A contact force is a type of force that is applied only when two objects are physically touching.

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Friction

Friction is a specific type of contact force that always opposes the motion between two surfaces.

Key Vocabulary

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Noncontact Force

A noncontact force is a push or pull on an object without direct physical contact.

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Gravity

Gravity is a fundamental noncontact force that attracts any two objects with mass toward each other.

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Net Force

Net force is the overall force acting on an object when all individual forces are added together.

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Mass

Mass measures the amount of matter, or 'stuff,' that makes up a physical object.

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Balanced Forces

Balanced forces are equal in size and opposite in direction, resulting in no change in motion.

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Unbalanced Forces

Unbalanced forces are not equal, causing a change in the object's speed or direction of motion.

Understanding Motion

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

• A reference frame is a fixed object used for comparison.

• Because of reference frames, all motion is considered relative.

Multiple Choice

Motion is defined as a change in an object's position compared to...

• its speed.

• a reference point.

• its mass.

• the time of day.

Multiple Choice

What is the function of a reference frame in determining motion?

• It increases an object's speed.

• It provides a fixed object for comparison.

• It measures an object's mass.

• It is the source of an object's energy.

Multiple Choice

A person standing on a sidewalk sees a car drive by. From the driver's perspective, the person on the sidewalk appears to be moving. Which statement best explains why both observations are correct?

• Only the car is truly in motion.

• The person on the sidewalk is creating an illusion of movement.

• Motion is relative because it depends on the observer's reference frame.

• The car's speed is too fast to be measured accurately.

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Types of Forces

Contact Forces

• A force is a push or a pull, defined by its strength in Newtons and its direction.

• Contact forces require objects to be in direct physical contact to be applied to one another.

• Friction is a contact force that opposes motion when two surfaces rub against each other.

Non-contact Forces

• A non-contact force is one that is applied to an object without physically touching it.

• Gravity is a non-contact force that pulls objects toward each other because of their mass.

• Magnetism and electrical forces are other key examples of non-contact forces that act at a distance.

Multiple Choice

What is the primary difference between contact and non-contact forces?

• Whether the objects must be physically touching to exert the force.

• The strength of the force being applied in Newtons.

• The direction in which the force is pushing or pulling.

• Whether the force is caused by mass or by motion.

Multiple Choice

When a book is slid across a table, how does the force of friction act on it?

• It is a contact force that opposes the motion of the book.

• It is a non-contact force that pulls the book downward.

• It is a contact force that helps the book speed up.

• It is a non-contact force that is measured in Newtons.

Multiple Choice

An apple falls from a tree and rolls on the ground before coming to a stop. Which statement best explains the forces involved in this entire process?

• Gravity, a non-contact force, pulls the apple down, and friction, a contact force, slows it down.

• Magnetism, a non-contact force, pulls the apple down, and gravity, a contact force, slows it down.

• Friction, a non-contact force, pulls the apple down, and gravity, a contact force, slows it down.

• Gravity, a contact force, pulls the apple down, and friction, a non-contact force, slows it down.

Sum of Forces (Net Force)

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

• Balanced forces result in a net force of zero and no change in motion.

• Unbalanced forces cause a change in an object’s motion, called acceleration.

Multiple Choice

What is the definition of net force?

• The sum of all forces acting on an object

• The force that causes an object to stop

• The single strongest force on an object

• The force of gravity acting on an object

Multiple Choice

What is the effect on an object's motion when the forces acting on it are balanced?

• The object begins to accelerate.

• There is no change in the object's motion.

• The object changes its direction.

• The object immediately stops moving.

Multiple Choice

If the forces acting on an object are unbalanced, what outcome can be predicted?

• The object will accelerate.

• The object will not move.

• The object will move at a constant speed.

• The object's motion will not change.

Mass and Changes in Motion

• Change in motion is related to mass and the net force applied.

• More massive objects need more force to change their motion.

• Applying a larger force results in a greater change in motion.

Multiple Choice

Which two factors are directly related to an object's change in motion?

• The object's size and its color

• The object's mass and the net force applied to it

• The object's temperature and its shape

• The object's speed and its direction

Multiple Choice

What is the relationship between an object's mass and the force needed to produce a change in its motion?

• A more massive object requires a greater force to change its motion.

• An object's mass has no effect on its change in motion.

• A less massive object requires a greater force to change its motion.

• Force is only required for objects that have no mass.

Multiple Choice

If a person applies the exact same amount of force to a bowling ball and a soccer ball, which outcome is most likely?

• The bowling ball will have a greater change in motion.

• The soccer ball will have a greater change in motion.

• Both objects will have the same change in motion.

• Neither object will change its motion.

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Common Misconceptions

Misconception	Correction
If an object is not moving, there are no forces acting on it.	An object at rest has balanced forces, resulting in a net force of zero.
A constant force is needed to keep an object moving.	An object in motion stays in motion unless a force like friction acts on it.
Motion is absolute.	Motion is always relative to a chosen frame of reference.

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Summary

• An object’s motion is a change in position from a reference frame.

• Forces are pushes or pulls, either contact (friction) or non-contact (gravity).

• Balanced forces cause no motion change; unbalanced forces cause acceleration.

• Acceleration depends on the object's mass and the net force applied.

Poll

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

• 1

• 2

• 3

• 4