Chapter Introduction

Lesson 1 Motion

Lesson 2 Forces

Lesson 3 Types of Forces

Lesson 4 Newton’s Laws

Chapter Wrap-Up

How do forces change the motion of objects?

Essential Question

• How is an object’s motion related to speed, velocity, and acceleration?

Motion

Distance

Position

Motion

Motion

Speed

Velocity

Acceleration

• When you explain to your friend how to get to your house, you most likely will tell them direction – the way to go to get there.

• You most likely will tell them distance.

– Distance is the length between two places.

What is motion?

• Using direction and distance, can help you identify the position of something.

– Position is the location of an object compared with things around it.

What is motion? (cont.)

• A complete description of your position includes a distance, a direction, and a reference point – a starting point that can be used to locate a place or thing.

What is motion? (cont.)

• Motion is a change in an object’s position compared to fixed objects around it.

• The man in the boat is not in motion compared to his fishing pole. He is in motion compared to the buoy.

What is motion? (cont.)

• When riding your bike, in relation to the bike, you are not moving.

• However, the trees and houses you pass appear to move backward. This is known as apparent motion – when things appear to an observer to be moving but are not actually changing position.

– You use apparent motion to determine what direction and how fast you are going.

What is motion? (cont.)

• Speed is how fast an object’s position changes with time at any given moment.

• Units of speed are expressed in units of distance per unit of time.

– Ex. Meters per second (m/s) or miles per hour (mph).

What is speed?

• Constant speed is a change of position in which the same distance is traveled each second.

– Ex. Look at the figure below. The same distance is traveled in the same amount of time.

What is speed? (cont.)

• Most moving objects do not travel at the same speed at all times.

– Ex. In a car, you are stopping, starting and passing other cars.

• When the car’s speed changes, it moves a different distance each period of time.

What is speed? (cont.)

• Because objects do not travel at the same speed at all times, it is useful to calculate an average speed.

• Average speed is the total distance traveled divided by the total amount of time.

What is speed? (cont.)

• Graphs that show comparisons between distance and time are called distance-time graphs.

• Constant speed is shown as a straight line on a distance-time graph.

What is speed? (cont.)

• You can use distance-time graphs to compare the motion of two different objects.

• The steeper line indicates a faster speed.

What is speed? (cont.)

• When an object is not traveling at constant speed, the graph is not a straight line but a curved line.

What is speed? (cont.)

• Velocity is a description of a moving object’s speed and direction.

– Ex. If a is car traveling south with a speed of 70mph, its velocity would be 70mph south.

What is velocity?

• The velocity of an object can be represented by an arrow.

• A greater speed is shown by a longer arrow.

• The arrow points in the direction of the object’s movement.

What is velocity? (cont.)

What is velocity? (cont.)

• Velocity changes when:

– the speed of an object changes

– the direction that the object moves changes

– or both the speed and the direction change

• Speed and velocity are often confused for the same thing but they are not.

– Speed tells how fast something is going.

– Velocity tells how fast something is going and in what direction.

What is velocity? (cont.)

• Acceleration is a change in the velocity of an object over time.

• An object accelerates when its velocity changes as a result of:

– increasing speed.

– decreasing speed.

– or a change in direction.

What is acceleration?

• Acceleration has a direction and can be represented by an arrow.

• The length of the arrow indicates the amount of acceleration.

• The direction of the arrow depends on whether velocity increases or decreases.

What is acceleration? (cont.)

• Acceleration is a change in velocity divided by the time interval during which the velocity changes.

What is acceleration? (cont.)

• When an object speeds up, the acceleration is in the same direction as the motion and is positive.

– Ex. Object goes from 0m/s to 6m/s in 3 sec.

acceleration = (6-0) ÷ 3

= 6 ÷ 3

= 2m/s2

What is acceleration? (cont.)

• When an object slows down, the acceleration is in the opposite direction as the motion and is negative.

– Ex. Object goes from 6m/s to 0m/s in 3 sec.

acceleration = (0-6) ÷ 3

= -6 ÷ 3

= -2m/s2

• Which could be stated as deceleration at 2m/s2

What is acceleration? (cont.)

• A speed-time graph shows how speed changes over time.

• A speed-time graph has time on the horizontal axis—the x-axis, and speed on the vertical axis—the y axis.

How can you graph acceleration?

• An object at rest is not moving, so its speed is always zero.

How can you graph acceleration? (cont)

• For an object moving at constant speed, the speed-time graph is a horizontal line.

How can you graph acceleration? (cont)

• The line on the speed-time graph for an object that is speeding up has an upward slope.

How can you graph acceleration? (cont)

How can you graph acceleration? (cont)

• The line on the speed-time graph for an object that is slowing down has a downward slope.

A. speed

B. constant speed

C. instantaneous speed

D. average speed

The total distance traveled divided by the total time taken to travel that distance is known as which of these?

A. a wavy line

B. a crooked line

C. a straight line

D. a curved line

How is constant speed represented on a distance-time graph?

A. 3km/h, upward

B. 3mph, upward

C. 3 km/h

D. 3km/h/h

A hot-air balloon rises at a rate of 3km/h. What is its velocity?

Essential Question

• What effect do forces have on objects?

Forces

Force

Contact force

Noncontact force

Forces

Net force

Balanced forces

Unbalanced forces

• Force is a push or pull exerted by one object on another, possibly causing a change in motion.

• Forces can cause:

– A moving object to accelerate or decelerate.

– An object to start moving.

– An object to change direction.

– An object to stop moving.

What are forces?

• Contact force is a force that is exerted only when objects are touching.

– Ex. baseball bat hitting a baseball or

fingers striking keyboard of computer

• Noncontact force is a force exerted when objects are not touching.

– Ex. Magnetic force, like a compass needle or

gravitational force, Earth around the Sun

What are forces? (cont.)

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

• 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.

How do forces affect each other?

• A force moving in the reference direction is positive, and a force in the opposite direction is negative.

• When the forces applied to an object act in the same direction, the net force is the sum of the individual forces.

How do forces affect each other? (cont.)

• 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.

How do forces affect each other? (cont.)

• Balanced forces are forces that act together on an object without changing its motion.

– Ex.

• Unbalanced forces are forces that do not cancel each other out and that cause an object to change its motion.

– Ex.

What are balanced and unbalanced forces?

A. balanced forces

B. inertia

C. net force

D. unbalanced forces

Which refers to forces that combine and form a net force that is not zero?

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?

A. color

B. mass

C. motion

D. weight

Which changes when unbalanced forces act on an object?

Essential Question

• What are the forces that act upon objects?

Types of Forces

Gravity

Friction

Air resistance

Types of Forces

Elastic force

Tension force

• Gravity is the force of attraction among all objects.

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

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

What is gravity?

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

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 its mass in kilograms.

What is gravity? (cont.)

• Friction is a force that opposes the motion of an object in contact with a surface.

• There are several types of friction.

– static friction

– sliding friction

– fluid friction

What is friction?

• Static friction prevents surfaces from sliding past each other.

– Ex. Keeps a book from starting to move across a table as a breeze blows through an open window.

– Up to a limit – An applied force can be strong enough to overcome some static friction.

What is friction? (cont.)

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

– Ex. Pushing a box across a floor.

• Rolling friction opposes the motion of a wheel turning along a surface.

– Ex. Skateboard and the street, slows the skateboarder down.

What is friction? (cont.)

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

– Air resistance the force that opposes the motion of an object through a gas.

• Ex. Leaves falling to the ground.

What is friction? (cont.)

• 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.

What is friction? (cont.)

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

What is friction? (cont.)

• Elastic force is a force exerted by a material when it is stretched or compressed.

• Tension force is a pulling force applied to an object that can make an object stretch.

– Ex. Stretching a rubber band.

The force from your fingers to pull the rubber band is a tension force. The force from the rubber band on your fingers is an elastic force.

What are elastic forces?

• Compression force is a squeezing force applied to an object that can make an object shrink. – Ex. Squeezing a rubber ball in your palm.

• The push back from the ball is an elastic force.

• Normal force is a force exerted by an object that is perpendicular to the surface of the object. – Ex. You sitting in a chair.

• Gravity is pulling you down

• Normal force from the chair is pushing up on you.

What are elastic forces? (cont.)

• Many forces can be acting on an object at the same time. These forces can also act in different directions.

– Ex. Sliding a book across a table.• There are both horizontal and vertical

forces acting on the book.

How can you identify forces on an object?

A. contact force

B. gravity

C. mass

D. weight

Which refers to gravitational force exerted on an object?

A. gravitational force

B. length

C. mass

D. noncontact force

Which is proportional to an object’s weight?

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

A. net force

B. lubricant

C. gravity

D. friction

Essential Question

• What are Newton’s three fundamental laws of motion?

Newton’s Laws

Newton’s first law of motion

Newton’s second law of motion

Newton’s third law of motion

Action force

Reaction force

Newton’s Laws

• Sir Isaac Newton studied how forces affect the motion of objects. He developed three rules known as Newton’s laws of motion.

What is Newton’s first law of motion?

• Newton’s first law of motion is a physical law stating that an object at rest tends to stay at rest and an object moving in a straight line at a constant speed tends to continue moving that way.

What is Newton’s first law of motion? (cont.)

• 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.

– Ex. Soccer ball still on the ground will stay there, because the vertical and horizontal forces are all balanced, until someone kicks it, then there is an unbalanced force acting on it.

What is Newton’s first law of motion? (cont.)

• Newton’s first law of motion is sometimes called the law of inertia.

• Inertia is the tendency of a moving object to keep moving in a straight line.

– Ex. If a crash test dummy is traveling in a test car and the brakes are pressed hard, what happens to the dummy?

• The dummy will be thrown forward because it is traveling at the same speed as the car. The unbalanced force that stopped the car was applied to the car and not the dummy.

What is Newton’s first law of motion? (cont.)

• Newton’s first law of motion shows that a net force is needed for an object to accelerate.

• Newton’s second law of motion shows how much acceleration this net force will cause.

What is Newton’s second law of motion?

• Newton’s second law of motion is a physical law that states that an object’s acceleration depends on the object’s mass and the amount of net force applied to it.

What is Newton’s second law of motion? (cont.)

• The direction of acceleration is the same as the direction of the net force.

• Acceleration is expressed in meters per second squared (m/s2), mass is in kilograms (kg), and force is in newtons (N).

What is Newton’s second law of motion? (cont.)

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

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

What is Newton’s second law of motion? (cont.)

• Momentum is useful for studying the motion of colliding objects.

– Ex. A train and a truck are traveling toward each other at the same speed. Which will have more momentum? Why?

• The train will have more momentum because it has more mass.

– Can the truck ever have more momentum than the train?

• Yes, if the truck’s speed is great enough.

What is Newton’s second law of motion? (cont.)

• 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.

What is Newton’s second law of motion? (cont.)

• Newton’s third law of motion relates forces between two objects.

• Newton’s third law of motion is a physical law that states that for every action there is an equal and opposite reaction.

– Ex. Space shuttle lifting off.

• Burning fuel produces hot gases. The shuttle pushes on the gases and the gases push on the shuttle causing it to lift off.

What is 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.

What is Newton’s third law of motion? (cont.)

• If the forces of a force pair always act in opposite directions and are always the same strength, why don’t they cancel each other?

– Because each force acts on a different object.

– Ex. The shuttle on the gas (action force)

The gas on the shuttle (reaction force)

What is Newton’s third law of motion? (cont.)

• Gravitational force exists among all objects in the universe.

• The force of gravity causes an astronaut to fall towards Earth but at the same time he is moving forward and no force is slowing down the acceleration.

– This is what keeps him/her on a circular path orbiting Earth and is why he/she does not feel the force of gravity or feels weightless.

What is weightlessness?

• True weightlessness is a term used to describe the condition of any object that has moved to a distant region and is in a position where there is no detectable gravitational pull.

• However, there is no place in the universe where gravitational pull does not exist and an object is truly without weight.

What is weightlessness? (cont.)

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?

Visual Summary

Chapter Review

Standardized Test Practice

An object’s motion changes if an unbalanced force acts on it.

• Speed is how fast an object’s position changes with time.

• Velocity changes when speed, direction, or both speed and direction change.

• Acceleration is a change in velocity over time. An object accelerates when it speeds up, slows down, or changes direction.

Lesson 1: Motion

Lesson 2: Forces

• A force is a push or a pull exerted by one object on another.

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

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

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

Lesson 3: Types of Forces

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

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

• Newton’s third law of motion states that forces come in pairs, and these two forces are equal in strength and opposite in direction.

Lesson 4: Newton’s Laws

A. contact

B. gravity

C. mass

D. noncontact

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

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. unbalanced

B. terminal

C. centripetal

D. balanced

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

A. It is positive.

B. It is negative.

C. The object is at rest.

D. The speed-time graph has a downward slope.

If acceleration is in the direction of motion, which of these is true?

A. The horizontal axis (the x-axis).

B. The vertical axis (the y-axis).

C. Speed is not indicated on a speed-time graph.

D. The speed is the slope of the line.

Where is speed located on a speed-time graph?

A. contact

B. gravitational

C. noncontact

D. static

Which describes the magnetic force?

A. displacement

B. reference point

C. motion

D. velocity

Which term refers to the process of changing position?

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?

A. the speed of an object changes

B. the direction an object moves changes

C. both the speed and direction of an object changes

D. all of the above

Which of these results in a change of velocity?

A. direction

B. speed

C. acceleration

D. velocity

Which term refers to a measure of how much the velocity of an object changes in a unit of time?