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Unit 4 Forces (Newton’s Laws)

The Nature of Forces

force A push or pull exerted on an object.

newton A unit of measure that equals the force required to accelerate 1 kilogram of mass at 1 meter per second per second.

net force The overall force on an object

when all the individual forces acting on it

are added together.

unbalanced forces Forces that produce a

nonzero net force, which changes an

object’s motion.

balanced forces Equal forces acting on an

object in opposite directions.

Newton’s First and Second Laws

inertia The tendency of an object to resist any change in its motion.

Newton’s Third Law

momentum The product of an object’s mass and velocity.

law of conservation of momentum The rule that in the absence of outside forces the total momentum of objects that interact does not change

Friction and Gravity

friction The force that one surface exerts on another when the two surfaces rub against each other.

static friction Friction that acts on objects that are not moving.

sliding friction Friction that occurs when one solid surface slides over another.

rolling friction Friction that occurs when an object rolls over a surface.

fluid friction Friction that occurs as an object moves through a fluid.

gravity The force that pulls objects toward each other.

mass The amount of matter in an object.

weight The force of gravity on an object at the surface of a planet.

free fall The motion of a falling object when the only force acting on it is gravity.

air resistance The fluid friction experienced by objects falling through the air.

terminal velocity The greatest velocity a falling object can achieve.

projectile An object that is thrown.



NEWTON’S FIRST AND SECOND LAWS

Guide for Reading 1. What is Newton’s first law of motion?

2. What is Newton’s second law of motion?

The English mathematician Sir Isaac Newton restated Galileo’s ideas about motion in the

first of his three laws of motion. Newton’s first law of motion states that an object at rest will

remain at rest, and an object moving at a constant velocity will continue moving at constant

velocity, unless it is acted upon by an unbalanced force.

Newton’s first law is also called the law of inertia. An object’s resistance to a change in

motion is called inertia. The amount of inertia an object has depends on its mass. Objects with

greater mass have more inertia, and require a greater force to cause a change in motion.

Newton’s second law of motion states that acceleration depends on the object’s mass

and the net force acting on the object. This relationship can be written in an equation.

𝐴𝑐𝑐𝑒𝑙𝑒𝑎𝑡𝑖𝑜𝑛 = 𝑁𝑒𝑡 𝐹𝑜𝑟𝑐𝑒

𝑀𝑎𝑠𝑠

When acceleration is measured in meters per second per second (m/s2) and mass is

measured in kilograms, force is measured in kilograms × meters per second per second

(kg·m/s2). This unit is called the newton (N). One newton equals the force required to

accelerate one kilogram of mass at 1 meter per second per second.

1 N = 1 kg × 1 m/s2

The acceleration of an object will increase if the force increases. According to the equation, acceleration and force change in the same way—both get larger. The equation also shows that the acceleration will increase if the mass decreases. Acceleration and mass change in opposite ways.

1 2 3 4 5 6 7



NEWTON’S FIRST AND SECOND LAWS

Guide for Reading and Study

The First Law of Motion

1. For an unmoving object to start moving, a(n) ________________________ has to act on it.

2. What is Newton’s first law of motion?

3. What is inertia?

4. What is another name for Newton’s first law?

5. The amount of inertia an object has depends on its

force

Newton’s first law states that an object at rest will remain at

rest, and an object moving at a constant velocity will continue

moving at a constant velocity unless it is acted upon by an

unbalanced force.

Inertia is an object’s resistance to change in its motion.

The other name for Newton’s first law is the law of inertia.

mass.

1

1

2

2

2



The Second Law of Motion

6. What is Newton’s second law of motion?

7. What is the equation that describes the relationship among the quantities of force, mass, and

acceleration?

8. Circle the letters of the two answers below that are different names for the same unit of

measure.

a. m/s2 b. N

c. kg · m/s2 d. 1 kg

9 How can you use Newton’s second law to find force?

10. What are two ways to increase the acceleration of an object?

Acceleration depends on the object’s mass and on the net

force acting on the object.

⃝ ⃝

Force = Mass x Acceleration

You can increase the acceleration of an object by increasing

the force acting on an object or by decreasing the mass of

the object.

Acceleration = Net force / Mass

3

4

5

4

7



Review and Reinforce

Newton’s First and Second Laws

Understanding Main Ideas

11. Newton’s second law of motion describes the relationship of force, mass, and acceleration.

Write the equation.

12. If you increase the force on an object, its acceleration

a. decreases. b. stays the same.

c. also increases. d. stops.

13. If you increase the mass on an object, its acceleration

a. decreases. b. stays the same.

c. also increases. d. stops.

14. How much force is needed to accelerate a 3 kg skateboard at 5 m/s2?

a. 8 N b. 0.6 N

c. 1.6 N d. 15 N

15. A resistance to a change in motion is

a. acceleration b. inertia

c. gravity d. velocity

16. The amount of inertia an object has depends on its

a. speed b. volume

c. mass d. length

Building Vocabulary Skills

17. Define the term inertia.

Acceleration = Net Force/Mass

⃝

⃝

⃝

⃝

⃝

Inertia is an object’s resistance to change in its motion.

4

7

7

4

2

2

2





NEWTON’S THIRD LAW

Guide for Reading 1. What is Newton’s third law of motion?

2. How can you determine the momentum of an object?

3. What is the law of conservation of momentum?

Forces are not “one-sided.” Whenever one object exerts a force on a second object, the second object exerts a force back on the first object. The force exerted by the second object is equal in strength and opposite in direction to the first force. The first force is called the “action” and the other force is the “reaction.” Newton’s third law of motion describes the relationship between these two forces. Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

Newton’s third law refers to forces on two different objects. The action and reaction forces described by this law cannot be added (combined) together because they are each acting on a different object. Forces can be added together only if they are acting on the same object.

All moving objects have momentum. Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object. The momentum of an object can by determined by multiplying the object’s mass and its velocity.

Momentum = Mass × Velocity

The unit for momentum is kilogram-meters per second (kg·m/s), since mass is measured in kilograms and velocity in meters per second. Like velocity and acceleration, momentum is described by its direction in addition to its quantity. The momentum of an object is in the same direction as the velocity of the object. The more momentum a moving object has, the harder it is to stop.

When two objects collide in the absence of friction, momentum is not lost. The law of conservation of momentum states that, in the absence of outside forces, the total momentum of the objects that interact does not change. It is the same before and after the interaction. The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. Friction would be an example of an outside force that might act on the objects. A quantity that is conserved is the same after an event as it was before the event.

Momentum is conserved when two objects, such as trains, collide. If one train traveling fast collides with a slower-moving train on the same track, the faster train slows down, and the slower train speeds up. If a moving train collides with a train at rest, the first train stops moving and the second train begins to move. If a moving train collides and locks with a train at rest, both cars will then move, but they will move more slowly than the first car did. In each of these examples, momentum is conserved.

1

2

3

4

5

6

7




Guide for Reading and Study

Newton’s Third Law of Motion

1. What is Newton’s third law of motion?

2. What is the name given to the force exerted by the first object on a second object?

3. What is the name given to the force exerted by the second object back on the first object?

4. The action and reaction forces in any situation will always be ________________________

and ________________________.

5. Explain why the equal action and reaction forces do not cancel each other when one person

hits a ball.

If one object exerts a force on another object, then the

second object exerts a force of equal strength in the

opposite direction on the first object.

action force

reaction force

opposite

Forces can be combined only if they are acting on the same

object. When one person hits a ball, the action force is

exerted on the ball, while the equal reaction

force is exerted back on the person.

Therefore, the equal forces in this

situation do not cancel each other.

equal

1

1

1

1

2



6. Explain why the equal action and reaction forces cancel each other when two people hit

volleyball in opposite directions.

Momentum

7. The product of an object’s mass and velocity is its

8. What is the equation you use to determine the momentum of an object?

9. What is the unit of measurement for momentum?

Conservation of Momentum

10. What does the law of conservation of momentum state?

momentum

Momentum = Mass x Velocity p = m • v

kilograms – meters per second kg • m/s

The total momentum of the objects that interact does not

change.

Forces can be combined together only if they are acting on the

same object. When two people hit a volleyball, the action

forces is exerted on the ball, while the equal reaction

forces is exerted back

on the two people.

Therefore, the equal

forces in this situation

do cancel each other.

2

4

5

3

6



11. Suppose a train car moving down a track at 10 m/s hits another train car that is not moving.

Explain how momentum is conserved after the collision.

Review and Reinforce



12. What does it mean to say that momentum is conserved?

13. How does the diagram illustrate Newton’s third law of motion?

In your answer, compare the force of the foot kicking the

soccer ball with the force of the soccer ball on the foot.

The result of the collision is that the first train car stops

and the second train car moves forward 10 m/s. In that

case, the momentum of the first train car is transferred to

the second train car so momentum is conserved.

It stays the same before and after an event.

Newton’s third law of motion says that if

an object, such as a foot, exerts a force

on another object, such as a soccer ball,

the second object (the soccer ball) exerts a force of equal

strength in the opposite direction on the first object (the

foot).

6

1

7



14. Could an elephant have the same momentum as a golf ball? Explain.

15. What is the momentum of a 20-kg dog running at a speed of 8 m/s?

16. Suppose you have two toy cars. Each has a mass of 0.04 kg. The cars have tape on their

bumpers that will cause them to couple (connect) together. One car is stopped on the track.

The other car, traveling at a velocity of 4 m/s, hits the first car. What is the momentum of the

coupled cars?


17. What is momentum?

18. Explain the law of conservation of momentum.

Since momentum equals mass times velocity, a very slow-

moving elephant could have the same momentum as a very

fast-moving golf ball.

20 kg x 8 m/s = 160 kg • m/s

The momentum of moving car = 0.04 kg x 4m/s = 0.16 kg • m/s

The momentum of the nonmoving car was zero. After the cars

couple (connect), the total momentum is 0.16 kg • m/s.

Momentum is

conserved.

The total momentum of a group of objects remains the same

unless outside forces act on the objects.

Momentum is mass times velocity.

4

3

7

3 or 4

6



Key Terms Forces Use the clues to help you find Key Terms hidden in the puzzle below. The terms may occur vertically,

horizontally, or diagonally.

1. A is a push or pull.

2. The overall force on an object after all forces are

added together is called the force.

3. The tendency of an object to resist change in its

motion is called .

4. The amount of matter in an object is called

.

5. One equals the force

required to accelerate 1 kilogram of mass at 1 meter

per second per second.

6. The force that one surface exerts on another when

the two rub against each other is called .

7. When solid surfaces slide over each other, the kind

of friction that occurs is friction.

8. The friction that occurs when an object moves

through a fluid is called friction.

9. The force that pulls objects toward Earth is

.

10. When the only force acting on a falling object is

gravity, the object is said to be in fall.

11. Objects falling through air experience a type of

fluid friction called resistance.

12. The force of gravity on a person or object at the

surface of a planet is known as .

13. The of an object is the

product of its mass and velocity.

14. Any object that travels around another object in

space is a(n) .

force

net

inertia

mass

newton

friction

sliding

friction

gravity

free

air

weight momentum

satellite

Documents

Unit 4 Forces (Newton’s Laws) - burnet.twpunionschools.orgburnet.twpunionschools.org/subsites/slama/documents/2016_files... · Unit 4 Forces (Newton’s Laws) ... © Pearson Education,