MS-PS2-2: Forces and Motion

MS-PS2-2
Forces and Motion

Middle School

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Learning Objectives

• Define force and identify examples like pushes, pulls, and gravity.

• Explain how an object's motion changes based on its mass and applied forces.

• Differentiate between balanced and unbalanced forces using the concept of net force.

• Describe how acceleration in an elevator affects an object's apparent weight.

Key Vocabulary

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Force

Force is an interaction, a push or a pull, that has the ability to change an object's motion.

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Gravity

Gravity is the universal force of attraction that exists between any two objects that possess mass.

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Newton (N)

The newton is the standard international unit used by scientists for measuring the strength of force.

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Net Force

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

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Balanced Forces

Balanced forces are equal in size and opposite in direction, resulting in a net force of zero.

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Unbalanced Forces

Unbalanced forces are not equal and opposite, causing a change in an object's speed or direction.

Key Vocabulary

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Mass

The amount of matter in an object, which also resists any changes in its motion.

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Inertia

An object's natural resistance to any change in its current state of motion.

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Newton's Second Law

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Reference Frame

A set of coordinates that is used to describe the motion of an object.

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Force as an Interaction

• A force is an interaction described as a push or a pull.

• It can start, stop, or change the direction of an object’s motion.

• Gravity is an invisible force pulling objects with mass toward each other.

• Balanced forces are equal and opposite, causing no change in an object's motion.

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Multiple Choice

What is the fundamental definition of a force?

• An interaction described as a push or a pull

• The speed of a moving object

• The amount of mass an object contains

• The energy an object has when it is not moving

Multiple Choice

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

• It can alter the object's speed or direction.

• It only affects objects that have no mass.

• It always makes the object move faster.

• It causes the object to become invisible.

Multiple Choice

A heavy dictionary is sitting motionless on a desk. Which statement best explains why it is not moving?

• The upward push of the table and the downward pull of gravity are balanced.

• There are no forces acting on the book, so it does not move.

• The force of gravity is pulling the book down, but the table has no force.

• The book has no mass, so gravity does not pull on it.

Measuring Force and Newton's First Law

• Force is measured in newtons (N) using a spring scale.

• An object's motion only changes when an outside force acts on it.

• A hockey puck stops moving on ice because of the force of friction.

Multiple Choice

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

• An outside force is required to change its motion.

• An object's mass must constantly decrease.

• The object must have a constant source of energy.

• The object must be in a vacuum.

Multiple Choice

Why does a hockey puck that is sliding on ice eventually slow down and stop?

• The puck runs out of its initial energy.

• The force of friction opposes the puck's motion.

• The cold temperature of the ice freezes the puck.

• The puck's weight pushes it down, stopping it.

Multiple Choice

If a hockey puck were sliding on a perfectly frictionless surface, what would most likely happen to its motion over time?

• It would continue to move at a constant speed and direction.

• It would gradually slow down as its energy runs out.

• It would speed up because there is no friction.

• It would stop immediately after the initial push.

Combining Forces: Net Force

• Net force is the total combination of all forces on an object.

• Balanced forces have a net force of zero, causing no change in motion.

• Unbalanced forces have a non-zero net force, causing the object to accelerate.

Multiple Choice

What is the definition of net force?

• The total combination of all forces acting on an object.

• The force of gravity pulling an object down.

• The speed at which an object is traveling.

• The mass of an object.

Multiple Choice

What is the relationship between balanced forces and an object's motion?

• The object begins to accelerate.

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

• The object's mass decreases.

• 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 remain stationary.

• The object will move at a constant velocity.

• The object's direction will not change.

The Role of Mass: Newton's Second Law

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

• Greater mass requires more force for the same change in motion.

• A larger force results in a greater change in motion (acceleration).

Multiple Choice

According to Newton's Second Law, what two factors determine an object's change in motion?

• Its mass and the net force applied to it

• Its speed and its direction of travel

• Its volume and its temperature

• Its shape and its texture

Multiple Choice

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

• A greater force is needed to cause the same change in motion.

• A smaller force is needed to cause the same change in motion.

• The object's mass has no effect on the required force.

• The object will not change motion regardless of the force.

Multiple Choice

If two shopping carts with different masses are pushed with the exact same force, what is the most likely outcome?

• The cart with less mass will have a greater change in motion.

• The cart with more mass will have a greater change in motion.

• Both carts will have the exact same change in motion.

• Neither cart will change its motion.

Frame of Reference

• All motion is described relative to a frame of reference.

• A bag on a train seems still to you, the passenger.

• To an outside observer, the same bag is moving with the train.

Multiple Choice

What is required to describe an object's motion?

• A frame of reference

• The object's total mass

• The force of gravity

• The time of day

Multiple Choice

Why does a bag on a train appear still to a passenger but moving to an observer standing outside the train?

• The passenger and the observer have different frames of reference.

• Only one of them is correctly observing the bag's motion.

• The bag is not actually moving with the train.

• The train is accelerating and decelerating.

Multiple Choice

Imagine you are riding on a bus that is moving at a constant speed. From which frame of reference would you appear to be in motion?

• A person sitting on a bench as the bus drives away.

• Another passenger sitting in the seat next to you.

• A backpack sitting on the floor of the bus.

• The driver of the bus.

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Planning an Investigation

• Plan your experiment to see how forces change an object's motion.

• The independent variable is what you change, like the force or mass.

• The dependent variable is what you measure, like the object's resulting acceleration.

• Keep other factors, called controls, constant, such as using the same mass.

Multiple Choice

In a scientific investigation, what is the role of the independent variable?

• It is the factor that is intentionally changed by the investigator.

• It is the factor that is measured to see the effect of a change.

• It is the factor that must be kept constant during the investigation.

• It is the final result of the investigation.

Multiple Choice

In an experiment studying how applied force affects an object's acceleration, what is the relationship between these two factors?

• The applied force is the independent variable, and the acceleration is the dependent variable.

• The acceleration is the independent variable, and the applied force is the dependent variable.

• Both force and acceleration are variables that should be kept constant.

• The mass is the dependent variable, and the force is a control.

Multiple Choice

A student wants to investigate how changing the applied force affects a cart's acceleration. To get a valid result, which of these must be kept constant?

• The mass of the cart

• The acceleration of the cart

• The force applied to the cart

• The final speed of the cart

Net Force and Apparent Weight

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At Rest/Constant Speed

• The upward push from the floor is equal to the force of gravity.

• The net force is zero because all the forces acting are balanced.

• You feel your normal weight because the net force is zero.

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Accelerating Downward

• The upward push from the floor is less than the force of gravity.

• This creates a net downward force, causing you to accelerate down.

• You feel lighter because the floor is not pushing up as hard.

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Accelerating Upward

• The upward push from the floor is greater than the force of gravity.

• This creates a net upward force, causing you to accelerate up.

• You feel heavier because the floor is pushing up on you harder.

Multiple Choice

What is true when the net force on an object is zero?

• The object is moving at a very high speed.

• The forces acting on the object are unbalanced.

• The force of gravity is the only force acting on the object.

• The forces acting on the object are balanced.

Multiple Choice

Why does a person feel lighter when accelerating downward?

• You feel heavier because the upward push from the floor is greater than gravity.

• You feel lighter because the upward push from the floor is less than gravity.

• You feel your normal weight because the forces are balanced.

• You feel no weight because there is no upward push from the floor.

Multiple Choice

If you are standing on a scale in an elevator and it shows a weight greater than your normal weight, what can you conclude about the elevator's motion?

• The elevator is accelerating downward.

• The elevator is moving at a constant speed.

• The elevator is accelerating upward.

• The elevator is at rest.

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Common Misconceptions

Misconception	Correction
If an object is not moving, there are no forces acting on it.	Stationary objects have balanced forces acting on them, like gravity and support force.
Mass and weight are the same thing.	Mass is the amount of matter in an object. Weight is the force of gravity on that mass.
A constant force is needed to keep an object moving at a constant speed.	A moving object will stay in motion without any force if there is no friction.

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Summary

• An unbalanced net force is required to change an object's motion.

• An object's acceleration depends on its mass and the net force applied.

• The description of any motion depends on the chosen frame of reference.

• Acceleration can cause your apparent weight to change.

Poll

On a scale of 1-4, how confident are you about calculating net force and predicting motion?

• 1

• 2

• 3

• 4