Physics Chapter 9 Flashcards
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?
Work; object’s energy
Work is required to lift a barbell. How many
times more work is required to lift the barbell three times as high?
Three
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?
Both the same, 200 J
How many joules of work are done on an
object when a force of 10 N pushes it a distance of 10 m?
10 N times 10 m = 100 J
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?
(100 J)/(0.5 s) 5 200 W
(100 J)/(1 s) 5 100 W
What are the two main forms of mechanical
energy?
PE and KE
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?
a. 100 J
b. 200 J
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?
200J
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?
16 times as much work;
16 times the distance
How does speed affect the friction between a road and a skidding tire?
Speed does not affect friction.
What will be the kinetic energy of an arrow having a potential energy of 50 J after it is shot from a bow?
50J
What does it mean to say that in any system the total energy score stay the same?
Energy is conserved.
In what two ways can a machine alter an input force?
It can change its magnitude
and/or direction
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?
Work out cannot exceed work
in; no.
What does it mean to say that a machine has a certain mechanical advantage?
It can multiply force by a
certain amount.
What is the efficiency of a machine that requires 100 J of energy to do 35 J of work?
35%
Distinguish between theoretical mechanical advantage and actual mechanical advantage.
How would these compare if a machine were 100% efficient?
TMA—no friction, AMA—with
friction; same
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?
10%
Calculate the work done when a force of 1 N moves a book 2 m.
2J
Calculate the work done when a 20-N force pushes a cart 3.5 m.
70J
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?)
PE= 1100J
Calculate the watts of power expended when a force of 1 N moves a book 2 m in a time interval of 1 s.
2W
Calculate the power expended when a 20-N force pushes a cart 3.5 m in a time of 0.5 s.
140W
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.)
40J, 80J
Calculate the increase in potential energy when a 20-kg block of ice is lifted a vertical
distance of 2 m.
400J
How much work is required to increase the kinetic energy of a car by 5000J?
5000J
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
2.5MJ
Which requires more work: stretching a strong spring a certain distance or stretching a weak spring the same distance?
More force to stretch strong
spring, so more work in stretching the same distance
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?
Same work is done by each,
for a same hour; climber in
30 s uses more power due to
shorter time.
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?
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
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?
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
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?
Energy is wasted as heat in a
non-hybrid car. In a hybrid
car, energy charges batteries
and is converted to electricity.
- 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.
PE at top 5 PE 1 KE.
Rearranging, KE 5 10,000 J 2
2000 J 5 8,000 J.
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?
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
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.)
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.
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
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.
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?
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.
