ch. 4 motion and forces - klein independent school...

Science Explorer Grade 7 Guided Reading and Study Workbook 37

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C H A P T E R 4

MOTION AND FORCES

The Nature of Force and Motion(pages 116-121)

This section explains how balanced and unbalanced forces are related to the motion ofan object. It also explains Newton’s first law of motion.

What Is a Force? (pages 116–117)

1. In science, a force is a push or a pull .

2. When one object pushes or pulls another object, the first object is

a force on the second object.

3. Circle the letters of the two ways that forces are described.

a. direction b. velocity c. strength d. acceleration

Unbalanced Forces and Motion (pages 117–118)

4. When two forces act in the same direction, they together.

5. Adding a force acting in one direction to a force acting in another

direction is the same as adding a(n) number and

a(n) number.

6. Look at Figure 1 on page 117. What does the width of the arrows tell you

about the forces they represent? The width of each arrow tells you the

strength of a force. A wider arrow shows greater force.

7. The overall force on an object after all the forces are added together is

called the .net force

negative

positive

add

exerting

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38 Guided Reading and Study Workbook Science Explorer Grade 7

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8. The illustrations to the right represent ways that two forces can combine. Draw lines from the left column to the right column to show the result of each combination.

CHAPTER 4, Motion and Forces (continued)

9. Unbalanced forces can cause an object to do three things. What are they?

Start moving, stop moving, or change direction

10. Is the following sentence true or false? Unbalanced forces acting on an

object will change the object’s motion.

11. Circle the letter of each sentence that is true about unbalanced forces.

a. When two forces act in opposite directions, the net force is thedifference between the two forces.

b. When two forces act in the same direction, the net force is thedifference between the two forces.

c. When two forces act in opposite directions, the net force is equal tothe greater force.

d. When two forces act in the same direction, the net force is the sum ofthe two individual forces.

Balanced Forces and Motion (page 118)

12. Equal forces acting on one object in opposite directions are called

.balanced forces

true

+ =0

+ =

+ =


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13. Is the following sentence true or false? Balanced forces acting on an

object will change the object’s motion.

14. When you add equal forces exerted in opposite directions, the net force

is .

Newton’s First Law of Motion (pages 120–121)

15. Is the following sentence true or false? Once an object is in its natural

resting place, it cannot move by itself.

16. Is the following sentence true or false? Galileo suggested that once anobject is in motion, no push or pull is needed to keep it moving.

17. What is inertia? Inertia is the tendency of an object to resist change in its

motion.

18. What is Newton’s first law of motion? An object at rest will remain at

rest, and an object that is moving at constant velocity will continue moving

at constant velocity unless acted upon by an unbalanced force.

19. Newton’s first law of motion is also called the law of .

20. What explains why you continue moving forward if you are in a car

that suddenly stops?

21. What is mass? Mass is the amount of matter in an object.

22. What is the SI unit of mass? the kilogram, or kg

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

24. How can mass be defined in terms of inertia? Mass can be defined as

a measure of the inertia of an object.

mass

inertia

inertia

true

true

zero

false


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Force, Mass, and Acceleration(pages 124-126)

This section explains how force and mass are related to the acceleration of an object.

Newton’s Second Law of Motion (pages 124–125)

1. What is Newton’s second law of motion? The net force acting on an

object is equal to the product of its acceleration and its mass.

2. What is the equation that describes the relationship among quantities offorce, mass, and acceleration?

Force = Mass � Acceleration

3. Circle the letters of two answers below that are ways to write units of force.

a. m/s2 b. N c. kg • ms2 d. 1 kg

4. Using Newton’s second law, write an equation you can use to findacceleration.

Changes in Force and Mass (page 126)

5. How does an increase of force affect acceleration? An increase of force

increases acceleration.

6. What are two ways you can increase the acceleration of an object?

You can increase the force or decrease the mass of the object.

Acceleration = ForceMass

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7. How does an increase of mass affect acceleration? An increase of mass

decreases the acceleration when forces remain the same.

8. Is the following sentence true or false? One way to increase the forceused to pull a wagon is to decrease the mass in the wagon.

Friction and Gravity(pages 127-133)

This section describes the effects of friction on surfaces that rub against each other. Italso describes how gravity acts between objects in the universe.

Friction (pages 128–129)

1. Is the following sentence true or false? When two surfaces rub together,the irregularities of one surface get caught on those of the other surface.

2. What is friction? Friction is the force that one surface exerts on another

when the two rub against each other.

3. Friction acts in a direction to the object’s directionof motion.

4. The strength of the force of friction depends on what two factors?

The types of surfaces involved and how hard the surfaces push together

5. How is friction useful in helping you walk? Friction acts between the

soles of your shoes and the floor. Without friction, your shoes would only

slide across the floor, and you would never move forward.

opposite

true

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true


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6. How does friction help an automobile move? An automobile moves

because of rolling friction between its tires and the road.

7. Complete the following table about the different kinds of friction.


Kinds of Friction

Kind of Friction Friction Occurs When . . .

Fluid friction An object moves through a fluid

Sliding friction Solid surfaces slide over each other

Rolling friction An object rolls over a surface

8. Which kind of friction requires more force to overcome, rolling friction

or sliding friction?

9. What kind of friction occurs when moving parts have ball bearings?

10. How does oil between machine parts reduce friction? The oil keeps the

machine parts from making direct contact, and there is fluid friction between

the parts instead of sliding friction.

Gravity (pages 130–132)

11. The force that pulls objects toward Earth is called .

12. When is an object said to be in free fall? An object is in free fall when

the only force acting on the falling object is gravity.

13. Near the surface of Earth, what is the acceleration of an object due to

the force of gravity?

14. An object that is thrown is called a(n) .projectile

9.8 m/s2

gravity

rolling friction

sliding friction

Airresistance

Gravity


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15. Is the following sentence true or false? An object that is dropped will hit

the ground before an object that is thrown horizontally.

16. Objects falling through air experience a type of fluid friction called

.

17. Is the following sentence true or false? The greater the surface area of

an object, the greater the air resistance.

18. On the diagram below, draw arrows that show the forces acting on thefalling acorn. Label each arrow with the name of the force.

true

air resistance

false

19. The greatest velocity a falling object reaches is called .

20. What is weight? Weight is the force of gravity on a person or object at the

surface of a planet.

21. How is weight different than mass? Weight is a measure of the force of

gravity on an object, while mass is a measure of the amount of matter in an

object.

22. Weight is usually measured in .newtons

terminal velocity


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Universal Gravitation (pages 132–133)

23. Is the following sentence true or false? The force that makes an applefall to the ground is the same force that keeps Earth orbiting the sun.

24. What does the universal law of gravitation state? The force of gravity

acts between all objects in the universe.

25. Is the following sentence true or false? On the moon, your mass would

be less than it is on Earth.

26. The force of attraction between two objects varies with what two

factors? Mass and the distance between the objects

Action and Reaction(pages 134-139)

This section explains Newton’s third law of motion. It also explains the law ofconservation of momentum.

Newton’s Third Law of Motion (pages 134–136)

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

2. What did Newton call the force exerted by the first object on a second

object? action force

SECTION

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false

true



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3. What did Newton call 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. Complete the flowchart below, which describes how a squid movesthrough water.

oppositeequal

reaction force

A squid expels water out its back end. This is the

force.action

The water expelled out of the back end of the squid pushes

back, exerting an equal and force

on the squid. This is the force.reaction

opposite

The squid moves through the

water as a result of the reaction force.

ahead

Newton’s Squid

6. Explain why the equal action and reaction forces do not cancel each

other when one person hits a ball. Forces can be added together 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 cannot be added

together and do not cancel each other.


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Momentum (page 137)

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?

Momentum = Mass � Velocity

9. What is the unit of measurement for momentum? kilogram-meters per

second, kg • m/s

Conservation of Momentum (pages 138–139)

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

momentum of the objects that interact does not change.

11. Suppose a train car moving down a track at 10 m/s hits another traincar that is not moving. Explain how momentum is conserved after the

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

second train car moves forward at 10 m/s. In that case, the momentum of

the first train car is transferred to the second train car.

momentum

A flowchart can help you remember the order in which a series of events occurs. Create aflowchart that describes how momentum is conserved when a moving train car collides withanother moving train car. See your textbook on page 138 . The first step in the flowchart will bethis: One train car moves down a track at 10 m/s. The last step in the flowchart will be this:Momentum is conserved. Do your work on a separate sheet of paper. For more informationabout flowcharts, see page 661 in the Skills Handbook of your textbook.

Reading Skill Practice

Students’ flowcharts will vary. A typical flowchart might include these steps: One train carmoves down a track at 10 m/s. The first train car collides with another train car, which is moving in the same direction at 5 m/s. During the collision, the first train car slows down to 5 m/s, and the second train car speeds up to 10 m/s. Momentum is conserved.


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Forces in Fluids(pages 140-150)

This section explains what causes pressure in fluids. It also explains Pascal’s principleand Bernoulli’s principle.

What Is Pressure? (pages 140–141)

1. What do snowshoes do that makes it easier for the person wearing them

to travel in deep snow? Snowshoes distribute a person’s weight over the

large area of the snowshoes, resulting in less downward pressure on the

snow compared to regular shoes. With less downward pressure, the person

doesn’t sink into the snow.

2. Is the following sentence true or false? Force and pressure are the same

thing.

3. What is pressure equal to? The force exerted on a surface divided by the

total area over which the force is exerted

4. Circle the letter of the term that is an SI unit of pressure.

a. newton b. liter

c. weight d. pascal

5. Circle the letter of the two answers below that are equal to each other.

a. 1 Pa b. 1 N/cm2

c. 1 N/m2 d. 1 N

6. Is the following sentence true or false? You can produce a lower

pressure by decreasing the area a force acts on. false

false

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Fluid Pressure (page 142)

7. A substance that can flow easily is a(n) .

8. Circle the letter of each of the following that are fluids.

a. helium gas b. liquid water c. ice d. air

9. Describe how molecules move in fluids. In fluids, molecules are

constantly moving in all directions. They are constantly colliding with each

other and with surfaces around them.

10. What causes the pressure exerted by a fluid? All of the forces exerted by

the individual molecules in a fluid add together to make up the pressure

exerted by the fluid.

11. The pressure exerted by a fluid is the total force exerted by the fluid

divided by the over which the force is exerted.

Pascal’s Principle (page 143)

12. What happens to the pressure in a bottle of water if you press the

stopper at the top down farther? The pressure in the water increases

everywhere in the bottle.

13. What is the relationship known as Pascal’s principle? When force is

applied to a confined fluid, an increase in pressure is transmitted equally to

all parts of the fluid.

Equilibrium in a Volcano (page 144)

14. Is the following sentence true or false? A volcano that is in equilibrium

erupts. false

area

fluid


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Force Pumps and Blood Flow (page 144)

15. What does a force pump do? It causes a fluid to move from one place to

another by increasing the pressure in the fluid.

16. Describe the heart in terms of force pumps. The heart is a force pump.

First, the heart pumps blood to the lungs. Then the blood returns to the

heart and is pumped to the rest of the body.

Using Pascal’s Principle (page 145)

17. Suppose you push down on a small piston that is connected to aconfined fluid, and another piston with the same area is connected by aU-shaped tube to the confined fluid. How much fluid pressure will the

second piston experience compared to the first? Both pistons will

experience the same fluid pressure.

18. In a hydraulic system, how is the force applied on a small surface area

multiplied? The increase in pressure on a small surface area is

transmitted to another part of a confined fluid, which pushes on a larger

surface area.

Pascal’s Principle in Nature (page 148)

19. The tube feet of a sea star take advantage of what principle to move

around?

20. When a sea star contracts different muscles, it changes the

in the fluid of its tube foot.

21. The a sea star exerts on the fluid in its systemcauses the tube foot to either push down or pull up on its sucker.

pressure

pressure

Pascal’s principle


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Bernoulli’s Principle (pages 148–150)

22. Is the following sentence true or false? The faster a fluid moves, the

more pressure the fluid exerts.

23. What does Bernoulli’s principle state? The pressure exerted by a moving

stream of fluid is less than the pressure of the surrounding fluid.

24. Is the following sentence true or false? A faster-moving fluid exerts less

pressure than a slower-moving fluid.

25. On the illustration of a wing below, draw arrows that show the path ofair above and below the wing.

26. Air that moves over the top of an airplane wing must travel fartherthan air that moves along the bottom of the wing. As a result, the air

moving over the top exerts less than the airmoving along the bottom.

27. What is lift? Lift is an upward force on a wing created by a difference in

pressure.

28. How do differences in air pressure cause smoke to rise up a chimney?

Wind blowing across the top of a chimney lowers the air pressure there. The

higher pressure at the bottom then pushes air and smoke up the chimney.

pressure

true

false

Direction of motion

Wing


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WordWiseUse the clues to help you find the key terms from Chapter 4 hidden in the puzzle below. Theterms may occur vertically, horizontally, or diagonally.

1. A is a push or pull.

2. The overall force on an object after allforces are added together is called the

force.

3. The tendency of an object to resistchange in its motion is called

.

4. One equals the forcerequired to accelerate 1 kilogram ofmass at 1 meter per second per second.

5. The force that one surface exerts onanother when the two rub against each

other is called .

6. When solid surfaces slide over eachother, the kind of friction that occurs is

friction.

7. A substance that can flow easily is

called a(n) .

8. The force that pulls objects toward

Earth is .

9. When the only force acting on a fallingobject is gravity, the object is said to be

in fall.

10. Objects falling through air experience atype of fluid friction called

resistance.

11. The of an objectis the product of its mass and velocity.

12. The force exerted on a surface dividedby the area over which the force is

exerted is called .pressure

momentum

air

free

gravity

fluid

sliding

friction

newton

inertia

net

force

s q m o m e n t u m

m f g i n e r t i a

a o r l i o l n g f

s r a i q a z y n r

w c v p c f r e e i

g e i w h t a e w c

u p t f l u i d t t

i e y c n i r o o i

n s l i d i n g n o

p r e s s u r e e n


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MathWiseFor the problems below, show your calculations on another sheet of paper. Write the answersfor the problems on the lines below.

Newton’s Second Law of Motion (pages 124–126)

1. Force = 65 kg � 3ms2 =

2. A 250-kg trailer is being pulled by a truck. The force causes the trailer toaccelerate at 4 m/s2. What is the net force that causes this acceleration?

Force = 250 kg � 4 m/s2 = 1,000 N

Answer:

Weight and Mass (page 132)

3. Weight = 45 kg � 9.8 m/s2 =

4. What is the weight of a rock that has a mass of 7 kg?

Weight = 7 kg � 9.8 m/s2 = 68.6 N

Answer:

Momentum (page 137)

5. Momentum = 5 kg � 6.5 m/s =

6. A baseball (mass = 0.14 kg) travels at 7 m/s, while a basketball (mass =0.5 kg) moves at 3 m/s. Which has the greater momentum?

The baseball’s momentum is 0.98 kg • m/s, while the basketball’s momentum

is 1.5 kg • m/s. The basketball has the greater momentum.

Calculating Pressure (page 141)

1. Pressure = =

2. A force of 25 N is exerted on a surface with an area of 5 m2. What is thepressure on that area?

Answer: Pressure = 5 Pa

Pressure = 25 N = 5 Pa5 m2

2 Pa20 N10 m2

32.5 kg • m/s

Weight = 68.6 N

441 N

Force = 1,000 N

195 N


ch. 4 motion and forces - klein independent school...

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