Physics Chapter 9

Question 1

A force sets an object in motion. When the
force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force distance, and what quantity can this change?

Answer 1

Work; object’s energy

Question 2

Work is required to lift a barbell. How many
times more work is required to lift the barbell three times as high?

Answer 2

Three

Question 3

Which requires more work, lifting a 10-kg
load a vertical distance of 2 m or lifting a
5-kg load a vertical distance of 4 m?

Answer 3

Both the same, 200 J

Question 4

How many joules of work are done on an
object when a force of 10 N pushes it a distance of 10 m?

Answer 4

10 N times 10 m = 100 J

Question 5

How much power is required to do 100 J of
work on an object in a time of 0.5 s? How
much power is required if the same work is
done in 1 s?

Answer 5

(100 J)/(0.5 s) 5 200 W
(100 J)/(1 s) 5 100 W

Question 6

What are the two main forms of mechanical
energy?

Answer 6

PE and KE

Question 7

a. If you do 100 J of work to elevate a bucket
of water, what is its gravitational potential
energy relative to its starting position?
b. What would the gravitational potential
energy be if the bucket were raised twice
as high?

Answer 7

a. 100 J
b. 200 J

Question 8

A boulder is raised above the ground so that its potential energy relative to the ground is 200 J. Then it is dropped. What is its kinetic energy just before it hits the ground?

Answer 8

200J

Question 9

Suppose you know the amount of work the brakes of a car must do to stop a car at a given speed. How much work must they do to stop a car that is moving four times as fast? How will the stopping distances compare?

Answer 9

16 times as much work;
16 times the distance

Question 10

How does speed affect the friction between a road and a skidding tire?

Answer 10

Speed does not affect friction.

Question 11

What will be the kinetic energy of an arrow having a potential energy of 50 J after it is shot from a bow?

Answer 11

50J

Question 12

What does it mean to say that in any system the total energy score stay the same?

Answer 12

Energy is conserved.

Question 13

In what two ways can a machine alter an input force?

Answer 13

It can change its magnitude
and/or direction

Question 14

In what way is a machine subject to the law of energy conservation? Is it possible for a machine to multiply energy or work input?

Answer 14

Work out cannot exceed work
in; no.

Question 15

What does it mean to say that a machine has a certain mechanical advantage?

Answer 15

It can multiply force by a
certain amount.

Question 16

What is the efficiency of a machine that requires 100 J of energy to do 35 J of work?

Answer 16

35%

Question 17

Distinguish between theoretical mechanical advantage and actual mechanical advantage.
How would these compare if a machine were 100% efficient?

Answer 17

TMA—no friction, AMA—with
friction; same

Question 18

What is the efficiency of her body when a cyclist expends 1000 W of power to deliver mechanical energy to the bicycle at the rate of 100 W?

Answer 18

10%

Question 19

Calculate the work done when a force of 1 N moves a book 2 m.

Answer 19

2J

Question 20

Calculate the work done when a 20-N force pushes a cart 3.5 m.

Answer 20

70J

Question 21

Calculate the work done in lifting a 500-N barbell 2.2 m above the floor. (What is the
potential energy of the barbell when it is lifted to this height?)

Answer 21

PE= 1100J

Question 22

Calculate the watts of power expended when a force of 1 N moves a book 2 m in a time interval of 1 s.

Answer 22

2W

Question 23

Calculate the power expended when a 20-N force pushes a cart 3.5 m in a time of 0.5 s.

Answer 23

140W

Question 24

How many joules of potential energy does
a 1-kg book gain when it is elevated 4 m?
When it is elevated 8 m? (Let g = 10 N/kg.)

Answer 24

40J, 80J

Question 25

Calculate the increase in potential energy when a 20-kg block of ice is lifted a vertical
distance of 2 m.

Answer 25

400J

Question 26

How much work is required to increase the kinetic energy of a car by 5000J?

Answer 26

5000J

Question 27

What change in kinetic energy does an airplane experience on takeoff if it is moved a distance of 500 m by a sustained by a sustained net force of 5000N

Answer 27

2.5MJ

Question 28

Which requires more work: stretching a strong spring a certain distance or stretching a weak spring the same distance?

Answer 28

More force to stretch strong
spring, so more work in stretching the same distance

Question 29

Two people who weigh the same amount climb a flight of stairs. The first person climbs the stairs in 30 s, while the second person climbs them in 40 s. Which person does more work? Which uses more power?

Answer 29

Same work is done by each,
for a same hour; climber in
30 s uses more power due to
shorter time.

Question 30

A physics teacher demonstrates energy conservation by releasing a heavy pendulum bob, as shown in the sketch,
allowing it to swing to and fro.
What would happen if, in his
exuberance, he gave the bob a
slight shove as it left his nose?

Answer 30

If ball is given an initial KE, it
returns to its starting position
with that KE (moving in the
other direction!) and hits the
instructor

Question 31

Consider the kinetic energy of a fly in the cabin of a fast-moving train. Does it have the same or different kinetic energies relative to the train? Relative to the ground outside?

Answer 31

Just as motion is relative, KE is
also. The speed and KE of the
fly are different relative to
the the train and the ground

Question 32

When a driver applies brakes to keep a car going downhill at a constant speed and constant kinetic energy, the potential energy of the car decreases. Where does this energy
go? Where does most of it go in a hybrid vehicle?

Answer 32

Energy is wasted as heat in a
non-hybrid car. In a hybrid
car, energy charges batteries
and is converted to electricity.

Question 33

50. A stuntman on a cliff has a PE of 10,000 J.Show that when his potential energy is 2000 J, his kinetic energy is 8000J.

Answer 33

PE at top 5 PE 1 KE.
Rearranging, KE 5 10,000 J 2
2000 J 5 8,000 J.

Question 34

Relative to the ground below, how many joules of PE does a 1000-N boulder have at the top of a 5-m ledge? If it falls, with how much KE will it strike the ground? What will be its speed on impact?

Answer 34

PE 5 mgh 5 W ? h 5
(1000 N)(5 m) 5 5000 J;
KE 5 PE so 1/2mv2 5 mgh, or
v2 5 2gh, or v 5 √ 2gh 5
√ 2(10)(5) 5 √ 100 5 10 m/s

Question 35

A hammer falls off a rooftop and strikes the ground with a certain KE. If it fell from a roof that was four times higher, how would its KE of impact compare? Its speed of impact? (Neglect air resistance.)

Answer 35

Four times higher means four
times the PE, thus four times
the KE of impact. From
KE 5 1/2mv2, this means twice
the impact speed (because
4 5 22). This result can be
obtained from d 5 1/2gt2,
where falling from 4d takes
twice the time. Twice the time
at the same acceleration g
means twice the speed; v 5 gt.

Question 36

A car can go from 0 to 100 km/h in 10 s. If the engine delivered twice the power, how many seconds would it take

Answer 36

Twice the power means doing
twice the work in the same
time or the same work in half
the time. To achieve the same
change in speed (and same
change in KE) with twice the
power means the work can be
done in half the time, or 5 s.

Question 37

If a car traveling at 60 km/h will skid 20 m when its brakes lock, how far will it skid if it is traveling at 120 km/h when its brakes lock?

Answer 37

Twice the speed means four
times the KE, and four times
the work to reduce the KE to
zero. F is constant so d 5
80 m.