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NNHS Introductory Physics: MCAS Review
Packet #2
2. Conservation of Energy and Momentum
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
1.) Which of the following objects has the most
kinetic energy?
A. Toy car with a mass of 1 kg and a speed of
1 m/s.
B. Toy car with a mass of 1 kg and a speed of
5 m/s.
C. Real car with a mass of 1000 kg and a speed
of 1 m/s.
D. Real car with a mass of 1000 kg and a speed
of 5 m/s.
2.) Calculate the kinetic energy of a dog,
mass = 10 kg, running at a speed of 2 m/s.
A. 10 J
B. 20 J
C. 40 J
D. 80 J
My Answer and Explanation:
D. KE= 1/2 * mass * speed
squared.
Bigger mass and bigger speed
means bigger KE.
My Answer and Explanation:
B. KE=1/2*mass*speed squared
1/2*10*4=20J
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
3.) A person is sitting at rest at the top of the
biggest hill of a rollercoaster. If the person has
a weight of 600 N, and the hill is 30 m high,
what is the person’s gravitational potential
energy?
A. 18000 J
B. 20 J
C. 180000 J
D. 9000 J
4.) You hold a 0.5 kg mass 1 meter above the
ground. Its gravitational potential energy is
approximately:
A. .5 J
B. 5 J
C. 10 J
D. 50 J
My Answer and Explanation:
A. GPE=Weight*Height
GPE = 600 N * 30 m = 18000 J
My Answer and Explanation:
B. GPE=mass * 10m/s2 * height
GPE =.5 kg*10m/s2*1 m
GPE = 5 J
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
5.) Three different boxes are lifted to different heights. • Box X weighs 115 N and is lifted to 15 m. • Box Y weighs 210 N and is lifted to 10 m. • Box Z weighs 305 N and is lifted to 5 m. Which of the following statements best describes the boxes’ change in mechanical energy? A. Box X had the greatest change in mechanical energy. B. Box Z had the smallest change in mechanical energy. C. Boxes X and Y had the same change in mechanical energy. D. Boxes Y and Z had the same change in mechanical energy.
6.) Which one of the following objects has
mechanical energy (KE +GPE) that remains
constant?
A. A crate being lifted vertically upwards at a
constant velocity.
B. An apple in free-fall.
C. A car accelerating on a level (flat) highway.
D. A sky-diver falling to Earth with his
parachute open.
My Answer and Explanation:
B.
Box X: Change in GPE
= weight * height
= 115 N * 15 m
= 1725 J
Box Y: Change in GPE
= 210 N * 10 m
= 2100 J
Box Z: Change in GPE
= 305 N * 5 m
= 1525 J
My Answer and Explanation:
B. An apple in free-fall does
not have air resistance acting
on it. Therefore, as it falls
the GPE is converted to KE and
the total remains constant.
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
7.) An astronaut drops a 1.0 kg object and a 5.0
kg object on the Moon. Both objects fall a total
distance of 2.0 m vertically. Which of the
following best describes the objects after they
have fallen a distance of 1.0 m?
A. They have each lost kinetic energy.
B. They have each gained the same amount of
potential energy.
C. They have each lost the same amount of
potential energy.
D. They have each gained one-half of their
maximum kinetic energy.
8.) The figure below shows a wagon that
moves from point X to point Y.
Which of the following best describes the
wagon’s change in energy as it coasts from
point X to point Y?
A. The wagon has the same kinetic energy at
point Y and at point X.
B. The wagon has more kinetic energy at point
Y than at point X.
C. The wagon has the same gravitational
potential energy at point Y and at point X.
D. The wagon has more gravitational potential
energy at point Y than at point X.
My Answer and Explanation:
D. As the objects fall, their
PE is converted to KE. When
they have fallen 1.0m they are
halfway to the ground. So half
of their GPE has been converted
to KE. So they have gained half
of their KE.
My Answer and Explanation:
D. At point Y the wagon is
higher above the ground. Thus
it has more GPE than it did at
point X.
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
9.) At a weightlifting competition, two
competitors lifted the same weight to the same
height. The second competitor accomplished
the lift 2 seconds faster than the first
competitor. This demonstrated that the second
competitor had more
A. energy than the first.
B. inertia than the first.
C. power than the first.
D. work than the first.
10.) An archer pulls back the bowstring to
prepare to shoot an arrow as shown below.
She uses an average force of 40 N, moving the
bowstring 0.2 m. How much energy is stored
in the bow?
A. 8 J
B. 16 J
C. 24 J
D. 36 J
My Answer and Explanation:
C. The second competitor has
more power since the same
amount of work was done in less
time:
Power = Work/time.
My Answer and Explanation:
A. The amount of energy stored
is based on the work done.
W = F*d
= 40 N * 0.2 m = 8 J.
Correct Answer and Explanation:
Correct Answer and Explanation:
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NNHS Introductory Physics: MCAS Review
Packet #2
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
11.) The Watt is the unit for which quantity:
A. energy
B. work
C. force
D. power
12.) One Joule is equal to
A. One Watt
B. One kg•m/s
C. One Newton-meter
D. One Newton
My Answer and Explanation:
D. The Watt is the unit for
power.
My Answer and Explanation:
C. Work is measured in Newton-
meters. Energy is measured in
Joules. Work is a transfer of
energy. Thus you know that
Joules and Newton-meters are
the same.
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
13.) What is the mass of an asteroid with a
speed of 200 m/s and a momentum of
2,000 kg•m/s?
A. 10 kg
B. 1,800 kg
C. 2,200 kg
D. 400,000 kg
14.) A bowling ball with a mass of 8.0 kg rolls
down a bowling lane at 2.0 m/s. What is the
momentum of the bowling ball?
A. 4.0 kg • m/s
B. 6.0 kg • m/s
C. 10.0 kg • m/s
D. 16.0 kg • m/s
My Answer and Explanation:
A. p = m v, so
m = p / v
= (2000 kg m/s) / 200 m/s
= 10 kg
My Answer and Explanation:
D. p = m v
p = 8 kg * 2 m/s
p = 16 kg m/s
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
15.) A student is standing on a skateboard that
is not moving. The total mass of the student
and the skateboard is 50 kilograms. The
student throws a ball with a mass of
2 kilograms forward at 5 m/s. Assuming the
skateboard wheels are frictionless, how will the
student and the skateboard move?
A. forward at 0.4 m/s
B. forward at 5 m/s
C. backward at 0.2 m/s
D. backward at 5 m/s
16.) You are at an ice skating rink and are
gliding towards a friend who is initially at rest.
When you reach your friend, you grab your
friend around the waist and the two of you
continue gliding forward. Which one of the
following is true:
A. Your speed after the collision is greater than
your speed before the collision.
B. Your speed after the collision is the same as
your speed before the collision.
C. Your speed after the collision is smaller
than your speed before the collision.
D. Not enough information has been provided.
My Answer and Explanation: C. Here are two ways to answer:
Conceptual Reasoning: Due to
conservation of momentum, if
the ball is thrown forward, the
student will go backward. Since
the boy has much more mass he
will have much less speed in
order to have the same
momentum.
Mathematical Reasoning: The
total momentum in the system
must remain to be zero so the
momentum of the ball forward
must be equal and opposite to
the momentum of the boy:
pball = 2 kg (5 m/s) = 10 kg m/s
therefore pboy = -10 kg m/s =
(50 kg) v therefore v = (-10 kg
m/s)/(50 kg)
= -0.2m/s, or 0.2m/s backward.
My Answer and Explanation:
C. When you grab your friend,
your total momentum remains
constant but the amount of mass
that moves is increasing. Thus
your speed must be decreasing.
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
Broad Concept: The laws of conservation of energy and momentum provide alternate approaches
to predict and describe the movement of objects.
17.) An engineering student is gathering data on the motion of a model car traveling down a ramp. If energy is conserved, the potential energy of the car at the top of the ramp should equal the kinetic energy of the car at the bottom of the ramp. After the first trial, the student calculates that the kinetic energy at the bottom of the ramp is less than the potential energy at the top of the ramp. Which of the following can best explain this difference? A. The car gained a small amount of mass as it moved down the ramp. B. The student accidentally accelerated the car at the top of the ramp. C. The measured height of the ramp was less than the actual height. D. The student did not include the effect of frictional force in the calculation.
18.) Mike, who has a mass of 75 kg, is running north at 2.6 m/s. He accidentally collides with Tom, who has a mass of 125 kg and is not moving. Which of the following statements describes how much momentum each person has before the collision? A. Mike has a momentum of 130 kg • m/s north, and Tom has no momentum. B. Mike has a momentum of 195 kg • m/s north, and Tom has no momentum. C. Both Mike and Tom have a momentum of 130 kg • m/s north. D. Both Mike and Tom have a momentum of 195 kg • m/s north.
My Answer and Explanation:
D. Two ways to answer:
Rule out wrong choices:
Choices A, B, and C would all
result in more KE of the car at
the bottom of the ramp. KE
depends directly on mass and
velocity, and “A” and “B” would
increase those quantities.
Choice “C” would mean the car
lost more GPE, therefore having
more KE at the bottom.
Why D is right:
Friction would do negative
work, which would reduce the
kinetic energy of the car at
the bottom.
My Answer and Explanation:
B. We can calculate the
momentum each
Person has before the
collision:
Mike: p = m v = 75 kg * 2.6 m/s
= 195 kg m/s
Tom: p = m v = 125 kg * 0 m/s
= zero kg m/s
Correct Answer and Explanation:
Correct Answer and Explanation:
KEY
NNHS Introductory Physics: MCAS Review
Packet #2
KEY
NNHS Introductory Physics: MCAS Review
Packet #2 Practice: Open-response Question #1
BE SURE TO ANSWER AND LABEL ALL PARTS OF THE QUESTION.
Show all your work (diagrams, tables, or computations)
If you do the work in your head, explain in writing how you did the work.
The above diagram shows a simple roller coaster track and one roller coaster car. The car,
when full of passengers, has a mass of 500kg.
A.) If the first hill has a height of 20 meters, calculate the amount of work that must be done to get the full car to the top of the first hill.
B.) If there were no friction, where on this track would the car be going the fastest? Place an x on the spot on the diagram. Explain.
C.) In reality, there is friction on a roller coaster track. Explain why this means that the first hill must be the highest hill on a roller coaster track.
A.) The work done to get the car to the top of the first hill is equal to the amount of energy that the car has at
the top of the hill. The amount of energy at the top of
the first hill is GPE=mgh=500*10*20=100,000Joules
B.) If there were no friction, then the total amount of energy will remain constant. The car will be going the
fastest when it has the most KE. It has the most KE when it
has the least GPE. It has the least GPE at the lowest
height. It has the lowest height at the end. Thus it is
going the fastest at the end.
C.) Since there is friction, energy will continually be lost to heat and sound. Therefore the total amount of energy that
the car has will continually decrease. In order to make it
over a hill, a car needs both GPE and KE. If the total is
Start End
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NNHS Introductory Physics: MCAS Review
Packet #2 less, the height of a hill will need to be less that the
first hill in order to have any KE.
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NNHS Introductory Physics: MCAS Review
Packet #2 Practice: Open-response Question #2
BE SURE TO ANSWER AND LABEL ALL PARTS OF THE QUESTION.
Show all your work (diagrams, tables, or computations)
If you do the work in your head, explain in writing how you did the work.
The illustrations below show an air track with two carts before and after a collision. The mass
and the initial velocity of each cart are shown below.
The first cart slides on the air track and collides with the second cart. The two carts stick together
upon impact and move together along the track, as shown below.
a. What is the momentum of the first cart before it collides with the second cart? Show your
calculations and include units in your answer.
b. What is the momentum of the second cart before the collision? Show your calculations and
include units in your answer.
c. Describe two changes that could be made initially to either one or both carts that would result
in an increase in the momentum of the combined carts after the collision.
A.)
p mv
p 0.2 kg * 0.1m/s
p 0.02 kg m/s
B.)
p mv
p 0.3 kg * 0.050 m/s
p 0.015 kg m/s
C.) You could increase the mass of either one or both of the
carts. You could increase the initial velocity of either one or
both of the carts. Any one of these four changes would increase
Before Collision:
First Cart: mass = 0.20 kg and velocity = 0.10m/s
Second Cart: mass = 0.30 kg and velocity = 0.050m/s
KEY
NNHS Introductory Physics: MCAS Review
Packet #2 the total amount of momentum before the collision and thus the
total amount of momentum after the collision.
KEY
NNHS Introductory Physics: MCAS Review
Packet #2 Practice: Open-response Question #3
BE SURE TO ANSWER AND LABEL ALL PARTS OF THE QUESTION.
Show all your work (diagrams, tables, or computations)
If you do the work in your head, explain in writing how you did the work.
In the diagram below, the falling water turns the waterwheel. The turning waterwheel generates electricity.
The water moves slowly at point A and then falls rapidly past point B. a. Describe the changes in kinetic and gravitational potential energy of the water as it travels from point A to point B. b. Explain why not all of the energy of the moving water available at point A is captured by the waterwheel to generate electricity. c. Describe two ways the system can be changed so that more energy from the falling water is converted into electrical energy.
A.) As the water travels from point A to point B, its gravitational potential energy decreases. While falling
from A to B, its kinetic energy increases.
B.) The waterwheel does not capture all of the energy of the moving water at point A because some of the energy is
used to turn the wheel itself, and that energy is not
available to generate electricity. Also, it appears that
some of the water is splashing and spilling over the water
wheel, so that water would not transfer its energy to the
wheel. Also, some of the energy goes into turbulence and
heating of the water.
C.) To get more energy from the falling water converted into electrical energy, you could build a dam above the
wheel, raising the water level above point A, or move the
wheel lower, so the water would fall from a greater height.
You could build a sluice or channel that would direct all
KEY
NNHS Introductory Physics: MCAS Review
Packet #2 the water smoothly onto the wheel, reducing losses due to
spilling and turbulence. If the wheel is made less massive,
with less friction, then less of the water’s energy would
be used to make the wheel turn. Another possibility is to
make the paddles of the wheel more of a “U-shape” rather
than flat, which lengthens the time the water is in
contact, transferring more momentum to the wheel.