What Is Force

What Is Force

Middle School

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Learning Objectives

• Define force, mass, and friction and their roles in how objects move.

• Explain how balanced and unbalanced forces change an object's motion.

• Describe how mass and the sum of forces affect an object's motion.

• Describe how forces can transfer energy between interacting objects.

Key Vocabulary

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Force

A push or pull between objects that can cause a change in the object's motion.

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Gravity

A force of attraction between two objects that have mass, pulling them toward each other.

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Friction

A force that opposes the motion of an object when it comes in contact with another surface.

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Newton (N)

The standard unit used to measure force in the International System of Units (SI).

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Weight

The measure of the force of gravity acting on an object's mass at a particular location.

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Balanced Forces

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

Key Vocabulary

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Unbalanced Forces

Unbalanced forces are pushes or pulls that cause a change in an object's motion.

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Net Force

The net force is the total combination of all forces that are acting on an object.

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Mass

Mass is the measure of the amount of matter or 'stuff' contained inside a physical object.

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Reference Frame

A reference frame is a particular point of view that is used to describe an object's motion.

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What is a Force?

• A force is a push or a pull that one object exerts on another.

• To move an object that is at rest, a force must be applied to it.

• Forces can change the motion of an object that is already moving.

• A force can make an object go faster, slow down, or even stop completely.

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Multiple Choice

What is a force?

• A push or a pull on an object

• The speed of a moving object

• The mass of an object at rest

• The energy stored in an object

Multiple Choice

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

• It can change the object's motion.

• It always increases the object's mass.

• It can change the object's temperature.

• It only works on objects that are at rest.

Multiple Choice

A toy car is already rolling forward. If you give it a gentle push from behind, in the same direction it is moving, what is the most likely outcome?

• The car will stop immediately.

• The car will move backward.

• The car will speed up slightly.

• The car will stay at the same speed.

Invisible Forces: Gravity and Balance

• Gravity is an invisible force that pulls objects toward Earth’s center.

• Gravity pulls down, and the ground pushes up with equal force.

• When forces are balanced, an object’s motion does not change.

Multiple Choice

What is the primary role of the force of gravity?

• It pulls objects toward the center of the Earth.

• It pushes objects away from the ground.

• It causes an object's motion to change.

• It makes invisible forces visible.

Multiple Choice

What happens when the downward pull of gravity on an object and the upward push from the ground are equal?

• The object's motion does not change.

• The object begins to move faster.

• The object begins to move downward.

• The object changes its shape.

Multiple Choice

A book is sitting still on a table. What would most likely happen if the table could no longer provide an upward push equal to the pull of gravity?

• The book would start to move downward.

• The book would remain perfectly still.

• The book would start to move upward.

• The force of gravity on the book would increase.

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Changing Ideas: From Aristotle to Galileo

Aristotle's View

• The Greek philosopher Aristotle believed that the natural state for objects was rest.

• He thought that a force must be continuously applied to keep an object moving.

• His influential ideas were based on observation but not systematic experiments.

Galileo's Experiments

• Galileo Galilei challenged older ideas by conducting careful experiments with balls on ramps.

• He identified friction as the force between surfaces that slows objects down.

• Galileo concluded an object in motion stays in motion unless a force acts on it.

Multiple Choice

Which statement best describes Aristotle's view of motion?

• He believed that an object's natural state was rest.

• He believed that friction was a force that sped objects up.

• He believed that experiments were the best way to test ideas.

• He believed that objects in motion stay in motion.

Multiple Choice

How did Galileo's experiments with balls on ramps lead to a new understanding of motion?

• Galileo proved that a force must be continuously applied to keep an object moving.

• Galileo identified friction as the force that slows objects down.

• Galileo showed that the natural state for all objects was rest.

• Galileo agreed with Aristotle's ideas but wrote them down differently.

Multiple Choice

Imagine a ball is pushed on a perfectly smooth surface with no friction at all. What would Galileo predict would happen to the ball after the push?

• The ball would speed up over time because motion is its natural state.

• The ball would gradually slow down and stop, as Aristotle predicted.

• The ball would keep moving at a constant speed because no friction is acting on it.

• The ball would stop immediately after the push ends.

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Newton's First Law and Net Force

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Multiple Choice

According to Newton's First Law, what is required for an object's motion to change?

• An unbalanced force acts on it.

• The net force acting on it is zero.

• It is affected only by friction.

• Its forces are perfectly balanced.

Multiple Choice

Under what condition is the net force on an object equal to zero?

• When the object starts to slow down.

• When the forces acting on it are balanced.

• When there is an unbalanced force.

• When friction is the only force present.

Multiple Choice

A hockey puck sliding on a rink gradually slows down and stops. What can be concluded from this observation?

• The net force on the puck must be zero.

• An unbalanced net force is acting on the puck.

• The puck's motion will not change.

• No forces are acting on the puck at all.

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Force, Mass, and Motion

• An object's change in motion depends on its mass and the forces on it.

• The greater the mass, the more force is needed to change its motion.

• For a given object, a larger force causes a larger change in its motion.

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Multiple Choice

What two factors determine an object's change in motion?

• The object's size and its shape

• The object's mass and the forces acting on it

• The object's speed and its direction

• The object's color and its texture

Multiple Choice

What is the relationship between an object's mass and the force required to change its motion?

• An object with greater mass requires less force to change its motion.

• An object with greater mass requires more force to change its motion.

• An object's mass has no effect on the force needed to change its motion.

• Only the object's speed determines the force needed to change its motion.

Multiple Choice

If a bowling ball and a soccer ball are both pushed with the exact same force, which will have a larger change in motion and why?

• The bowling ball, because it is heavier and will roll farther.

• The soccer ball, because the same force will cause a larger change in motion for the object with less mass.

• Both will have the same change in motion because the force is the same.

• Neither will move because the force is not strong enough.

Describing Motion: Reference Frames

• A reference frame is a point of view to measure motion.

• An object's motion looks different from different reference frames.

• Scientists must agree on a reference frame to describe motion clearly.

Multiple Choice

What is a reference frame?

• A tool used to measure an object's mass

• A point of view from which to measure motion

• The path an object takes when it moves

• The speed and direction of an object

Multiple Choice

What happens to the observation of an object's motion if the observer's reference frame changes?

• The object's motion will always look the same

• The object's motion can appear to be different

• The object will immediately stop moving

• The object's mass will seem to change

Multiple Choice

Based on the principles of motion, why must scientists agree on a reference frame when communicating their findings?

• To prove that one point of view is correct and others are wrong

• To make sure the object moves at a constant speed

• To ensure descriptions of motion are clear and consistent

• To measure the object's motion without any tools

Forces and Energy Transfer

• Applying a force to an object can transfer energy to it.

• Pulling attracting magnets apart requires a force and transfers energy.

• Pushing repelling magnets together also requires a force and transfers energy.

Multiple Choice

What can happen when a force is applied to an object?

• Energy can be transferred to the object.

• The object's mass always increases.

• The object will always get hotter.

• Energy is removed from the object.

Multiple Choice

What is the key similarity between pulling attracting magnets apart and pushing repelling magnets together?

• Both require applying a force that transfers energy.

• Both cause the magnets to become weaker.

• Both demonstrate how energy creates a force.

• Both actions can only happen in a vacuum.

Multiple Choice

A person uses their hands to squeeze a spring, compressing it. Which statement best explains what has occurred?

• The force from the person's hands transferred energy to the spring.

• The spring lost energy because it was compressed.

• Compressing the spring destroyed the energy within it.

• Only magnets can have energy transferred to them by a force.

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Common Misconceptions

Misconception	Correction
If an object isn't moving, no forces are acting on it.	An object at rest can have multiple balanced forces. The net force is zero.
Constant force is needed for an object to move at a constant speed.	Without friction, no extra force is needed for constant motion (Newton's First Law).
Mass and weight are the same.	Mass is the amount of matter. Weight is the force of gravity on mass.

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Summary

• A force is a push or pull that can change an object's motion.

• Unbalanced forces change an object's motion, while balanced forces do not.

• An object's change in motion depends on its mass and the force applied.

• All motion is described from a reference frame and can transfer energy.

Poll

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

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