Chapter Introduction

Lesson 1 Newton’s First Law

Lesson 2 Gravity and Friction

Lesson 3 Newton’s Second Law

Lesson 4 Newton’s Third Law

Chapter Wrap-Up

How do forces change the motion of objects?

What do you think?

Before you begin, decide if you agree or disagree with each of these statements. As you view this presentation, see if you change your mind about any of the statements.

1. You pull on objects around you with the force of gravity.

2. Friction can act between two unmoving, touching surfaces.

3. Forces acting on an object cannot be added.

Do you agree or disagree?

4. A moving object will stop if no forces act on it.

5. When an object’s speed increases, the object accelerates.

6. If an object’s mass increases, its acceleration also increases if the net force acting on the object stays the same.

Do you agree or disagree?

7. If objects collide, the object with more mass applies more force.

8. Momentum is a measure of how hard it is to stop a moving object.

Do you agree or disagree?

• force

• contact force

• noncontact force

• gravity

Gravity and Friction

• mass

• weight

• friction

• According to Newton’s first law of motion, an object at rest will stay at rest, and an object in motion will continue moving unless acted upon by an unbalanced force.

Newton’s First Law of Motion

• A push or a pull is called a force.

• An object or a person can apply a force to another object or person.

Types of Forces

force

from Latin fortis, means “strong”

• Balanced forces forces on an object that are equal in size and opposite in direction . Net force = 0Do not cause motion.

• Forces that combine and form a net force >zero are unbalanced forces. Cause motion.

Identifying Forces (cont.)

• Balanced forces acting on an object do not change the object’s speed and direction.

• Newton’s first law of motion only applies to balanced forces acting on an object.

• When unbalanced forces act on an object, the object’s velocity changes.

Newton’s First Law of Motion (cont.)

• A contact force is a force that is applied when two objects touch.

• A force that one object can apply to another object without touching it is a noncontact force.

Types of Forces (cont.)

Can you think of an example of a contact force?

Can you think of an example of a noncontact force?

Types of Forces (cont.)

• Contact: Pushing a book off of a table

• Noncontact: Gravity or magnetic forces

• Agree or Disagree: A moving object will stop if there are no forces acting on it?

• Think About it: If you kick a soccer ball across a field will it go on forever?

• What forces will cause it to stop?

• If there where no forces acting on it (friction, gravity, walls, people) would the soccer ball continue to move?

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

• The net force depends on the directions of the forces applied to an object.

• Because forces have direction, you have to specify a reference direction when you add forces.

Identifying Forces

• When forces act in opposite direction on an object, the net force is still the sum of the forces.

• The net force is the sum of the positive and negative forces.

Identifying Forces (cont.)

• Inertia- the tendency of an object to resist changes in motion

• Examples…

• An object with less mass has less inertia and an object with more mass has more inertia

• Can you think of any examples?

Is it easier to throw a baseball or a bowling ball? Explain why.

 

Explain which has greater inertia: a jet plane sitting on the runway or a speeding car?

Types of Forces (cont.)

• Unbalanced forces cause an object to move.

• According to Newton’s first law of motion, if the net force on an object is zero, the object’s velocity does not change.

• Inertia is a property that resists a change in the motion of an object.

• Unbalanced forces cause an object to speed up, slow down, or change direction.

• Friction and gravity are two forces that can change the motion of an object.

• What is the law of universal gravitation?

• How does friction affect the motion of two objects sliding past each other?

Gravity and Friction2

• Friction is a force that opposes the movement between two touching surfaces.

• There are several types of friction.

• static friction

• sliding friction

• fluid friction

Friction

• What causes friction between surfaces?

– When the microscopic dips and bumps on one surface catch the dips and bumps on another surface, the microscopic roughness slows sliding.

Friction (cont.)

• Static friction prevents surfaces from sliding past each other.

• Up to a limit, the strength of static friction changes to match the applied force.

• Sliding friction opposes the motion of surfaces sliding past each other.

Friction (cont.)

• Fluid friction is friction between a surface and a fluid—any material, such as water or air, that flows.

• Fluid friction between a surface and air is air resistance.

• The greater the surface area of an object, the more air resistance it will experience.

Friction (cont.)

Friction (cont.)

How does friction affect the motion of two objects sliding past each other?

• Lubricants decrease friction and with less friction, it is easier for surfaces to slide past each other.

•What reduces friction between a door and a hinge?

•What reduced friction between two bones in a joint?

Reducing Friction

• Increasing Friction

– How would you increase the friction between your foot and the ice on the sidewalk?

• Gravity is an attractive force that exists between all objects that have mass.

• Objects fall to the ground because Earth exerts gravity on them.

• Earth’s gravitational force pulls objects towards Earth’s surface.

• Mass is the amount of matter in an object.

• Mass is often measured in kilograms (kg).

What is gravity?

• Sir Isaac Newton developed the law of universal gravitation in the late 1600s.

• The law of universal gravitation states that all objects are attracted to each other by a gravitational force.

What is gravity? (cont.)

• The strength of force depends on the mass of each object and the distance between them.

• When the mass of one or both objects increases, the gravitational force between them also increases.

What is gravity? (cont.)

• Weight is the gravitational force exerted on an object.

• Near Earth’s surface, an object’s weight is the gravitational force exerted on the object by Earth.

• Because weight is a force, it is measured in newtons.

What is gravity? (cont.)

• An object’s weight is proportional to its mass.

• Near Earth’s surface, the weight of an object in newtons is about ten times (9.8x) its mass in kilograms.

What is gravity? (cont.)

• For an object to start moving, a force greater than static friction must be applied to it.

• To keep an object in motion, a force at least as strong as friction must be applied continuously.

• Objects stop moving because friction or another force acts on them.

Why do objects stop moving?

• Gravity is an attractive force that acts between any two objects that have mass. The attraction is stronger for objects with greater mass.

• Friction can reduce the speed of objects sliding past each other. Air resistance is a type of fluid friction that slows the speed of a falling object.

LESSON 1 – LR1

A. contact force

B. gravity

C. mass

D. weight

Which refers to gravitational force exerted on an object?

LESSON 1 – LR2

A. gravitational force

B. length

C. mass

D. noncontact force

Which is proportional to an object’s weight?

1. You pull on objects around you with the force of gravity.

2. Friction can act between two unmoving, touching surfaces.

Do you agree or disagree?

A. Gravity

B. Newton

C. Force

D. Motion

A push or a pull is called a?

A. net force

B. lubricant

C. gravity

D. friction

Which is a force that opposes the movement between two touching surfaces?

A. balanced forces

B. inertia

C. net force

D. unbalanced forces

Which refers to forces that combine and form a net force that is greater than zero?

A. friction

B. inertia

C. unbalanced forces

D. velocity

Which could cause an object to stop moving?

A. zero

B. one

C. equal

D. balanced

When equal forces act on an object in opposite directions, what is the net force on the object?

2. Friction can act between two unmoving, touching surfaces.

Agree

3. Forces acting on an object cannot be added.

Disagree

Do you agree or disagree?

• What is Newton’s second law of motion?

• How does centripetal force affect circular motion?

Newton’s Second Law

• Forces change an object’s motion by changing its speed, its direction, or both its speed and its direction.

• Velocity is speed in a certain direction.

• Only unbalanced forces change an object’s velocity.

• Unbalanced forces make an object accelerate by changing its speed, its direction, or both.

How do forces change motion?

• Acceleration is a measure of how much the velocity of an object changes in a unit of time.

• An object accelerates when its velocity changes as a result of increasing speed, decreasing speed, or a change in direction.

Acceleration—Changes in Velocity

• When unbalanced forces act on an object at rest, the object begins moving

– in the same direction of the net force.

• If the net force acting on a moving object is in the same direction that the object is moving

– the object will speed up.

• If the direction of the net force on an object is opposite to the direction the object moves

– the object slows down.

How do forces change motion? (cont.)

According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on the object divided by the object’s mass.

Newton’s Second Law of Motion

• As the mass of an object increases the amount of force required to move the object:

– Increases

• As the force applied to an object increases the acceleration of the object will:

– Increase

• Force = Mass x Acceleration

• Acceleration is expressed in meters per second squared (m/s2),

• Mass in kilograms (kg),

• Force in Newtons (N).

Newton’s Second Law of Motion (cont.)

A. acceleration

B. centripetal force

C. inertia

D. velocity

Which term refers to speed in a certain direction?

A. inertia

B. acceleration

C. circulation motion

D. centripetal force

Which refers to a change in velocity over time?

5. When an object’s speed increases, the object accelerates.

Agree

6. If an object’s mass increases, its acceleration also increases if the net force acting on the object stays the same.

Disagree

Do you agree or disagree?

• The SI unit for force is the newton (N).

• Arrows can be used to show both the strength and direction of force.

• What is Newton’s third law of motion?

• Why don’t the forces in a force pair cancel each other?

• What is the law of conservation of momentum?

Newton’s Third Law

• Newton’s third law of motion

• force pair

• momentum

• law of conservation of momentum

Newton’s Third Law

According to Newton’s third law of motion, for every action there is an equal and opposite reaction.

•Ex: When you jump on a trampoline you exert a downward force on the trampoline and it exerts an upward force on you.

–The more force you use to jump down with, the higher you bounce back into the air.

Newton’s Third Law of Motion

• A force pair is the forces two objects apply to each other.

• In a force pair, one force is called the action force and the other force is called the reaction force.

Newton’s Third Law of Motion (cont.)

• When you push against an object, the force you apply is called the action force.

• The force applied by the object back against you is called the reaction force.

Using Newton’s Third Law of Motion

• Can you think of some examples that illustrate Newton’s 3rd law?

– Parachute- downward force of you falling, upward force of air

– Swimming pool- backward force of you pushing, forward force of you moving away from wall

• If the forces of a force pair always act in opposite directions and are always the same strength, they do not cancel each other out because each force acts on a different object.

• STOP HERE

• Discuss: What is momentum?

• Which object will have the most momentum (tennis ball, wiffle ball, or a lacrosse ball?)

Momentum is a measure of how hard it is to stop a moving object.

Momentum

momentum

from Latin momentum, means “movement, impulse”

• Read through the lab

• Answer the pre-lab questions

• What safety precautions should we take while completing this lab

Momentum (cont.)

Momentum is the product of an object’s mass and velocity.

http://connected.mcgraw-hill.com/

Momentum (cont.)

• An object’s momentum is in the same direction as its velocity.

• According to Newton’s first law, if the net force on an object is zero, neither its velocity nor its momentum change.

• Because momentum is the product of mass and velocity, the force on an object equals its change in momentum.

According to the law of conservation of momentum, the total momentum of a group of objects stays the same unless outside forces such as friction act on the objects.

Conservation of Momentum

What is the law of conservation of momentum?

• When colliding objects bounce off each other, it is an elastic collision.

• If objects collide and stick together, the collision is inelastic.

• No matter the type of collision, the total momentum will be the same before and after the collision.

Conservation of Momentum (cont.)

• Newton’s third law of motion describes the force pair between two objects.

• For every action force, there is a reaction force that is equal in strength but opposite in direction.

• In any collision, momentum is transferred from one object to another.

A. Newton’s third law of motion

B. Newton’s second law of motion

C. Newton’s first law of motion

D. law of conservation of momentum

Which law explains that every force has a reaction force in the opposite direction?

A. velocity

B. momentum

C. inertia

D. elastic collision

Which is a measure of how hard it is to stop a moving object?

A. velocity

B. momentum

C. force pair

D. collision

Which refers to the forces two objects apply to each other?

7. If objects collide, the object with more mass applies more force.

8. Momentum is a measure of how hard it is to stop a moving object.

Do you agree or disagree?

Key Concept Summary

Interactive Concept Map

Chapter Review

Standardized Test Practice

An object’s motion changes if a net force acts on the object.

• Friction is a contact force. Magnetism is a noncontact force.

• The law of universal gravitation states that all objects are attracted to each other by gravity.

• Friction can stop or slow down objects sliding past each other.

Lesson 1: Gravity and Friction

Lesson 2: Newton’s First Law

• An object’s motion can only be changed by unbalanced forces.

• According to Newton’s first law of motion, the motion of an object is not changed by balanced forces acting on it.

• Inertia is the tendency of an object to resist a change in its motion.

• According to Newton’s second law of motion, an object’s acceleration is the net force on the object divided by its mass.

• In circular motion, a centripetal force pulls an object toward the center of the curve.

Lesson 3: Newton’s Second Law

• Newton’s third law of motion states that when one object applies a force on another, the second object applies an equal force in the opposite direction on the first object.

• The forces of a force pair do not cancel because they act on different objects.

• According to the law of conservation of momentum, momentum is conserved during a collision unless an outside force acts on the colliding objects.

Lesson 4: Newton’s Third Law

A. weight

B. mass

C. inertia

D. gravity

Which term refers to the amount of matter in an object?

A. contact

B. gravity

C. mass

D. noncontact

Which refers to a force that is applied when two objects touch?

A. unbalanced

B. terminal

C. centripetal

D. balanced

Which term refers to forces that combine with a net force of zero?

A. velocity

B. terminal velocity

C. net force

D. inertia

The tendency of an object to resist a change in its motion is called what?

A. Newton’s third law of motion

B. Newton’s second law of motion

C. Newton’s first law of motion

D. law of conservation of momentum

Which law states that the total momentum of a group of objects stays the same unless outside forces act on the objects?

A. weight

B. noncontact

C. inertia

D. contact

Which term refers to a force that one object can apply to another without touching?

A. contact

B. gravitational

C. noncontact

D. static

Which describes the magnetic force?

A. unbalanced

B. proportional

C. noncontact

D. negative

Which describes the net force of different forces acting in the same direction?

A. stops

B. speeds up

C. slows down

D. changes direction

If the net force acting on a moving object is in the direction that the object is moving, the object does what?

Which refers to the collision of objects that bounce off each other?

A. elastic

B. friction

C. inelastic

D. inertia