Download Midyear Review 2014 KEY

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Introductory Physics: Midyear Review
1. Motion and Forces
Central Concept: Newton’s laws of motion and gravitation describe and predict the motion
of most objects.
1.1 Compare and contrast vector quantities (e.g., displacement, velocity, acceleration, force,
linear momentum) and scalar quantities (e.g., distance, speed, energy, mass, work).
1.!! Which of the following is a vector quantity?
2.!!Which of the following statements identifies the main
A. displacement
B. speed
C. mass
D. distance
difference between vector and scalar quantities?
A. Only vectors have direction.
B. Only scalars have magnitude.
C. Only scalars have units associated with them.
D. Only vectors have units associated with them.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: A
CORRECT ANSWER: A
Vectors are measurements that have both
magnitude and direction, whereas scalars have
only magnitude. Speed, mass and distance are
quantities with only magnitude. An example of
a displacement is “5m North” which specifies
both a magnitude and a direction, making it a
vector.
Only vectors have direction. This is the primary
difference between vectors and scalars, which
have only magnitude. All measurements have
units associated with them.
1.2 Distinguish between displacement, distance, velocity, speed, and acceleration. Solve
problems involving displacement, distance, velocity, speed, and constant acceleration.
3.!! The diagram below shows the path of a jet from
4.!!Jill drives from Newton to Boston. Her ending point
Washington, D.C. to Dallas, TX.
5.!! Which of the following cars is NOT accelerating?
A. A car that is slowing down to a stop at a red light.
B. A car that is traveling at a constant speed around a
sharp curve.
C. A car that is speeding up as it passes another car
on the highway.
D. None of the above. All of the cars described are
accelerating.
6.!!The following table describes the distance a toy car
travels.
Time(s)
0
1
2
3
Distance (cm)
0
5
10
?
The car travels at a constant velocity for the first 2
seconds. If the toy car starts accelerating, which of
the following could be the next distance in the table?
A. 0cm
B. 3cm
C. 12cm
D. 15cm
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: D
CORRECT ANSWER: C
Acceleration includes speeding up, slowing
down, and changing direction. A, B and C all
include one of these actions. D is the only
option with no change in speed or direction.
Distance is the how far the car has traveled in
total. As the car continues to travel, the
distance must increase, so A and B cannot be
the answers. If the car continues to travel at a
constant velocity it would travel a total distance
of 15 cm, but since the car is accelerating, (ie.
speeding up, slowing down, or turning) the
answer cannot be D, so the answer must be C
and the car must be slowing down. Whew.
This was not easy to explain.
1.3 Create and interpret graphs of 1-dimensional motion, such as position vs. time, distance vs.
time, speed vs. time, velocity vs. time, and acceleration vs. time where acceleration is constant.
7.!! The graph below relates distance to time for a rolling 8.!!The graph below shows a beetle’s movement along a
ball.
plant stem.
What is the average speed of the ball?
A. 2 m/s
B. 6 m/s
C. 8 m/s
D. 72 m/s
During which span of time was the beetle not
moving?
A. from 0 to 4 minutes
B. from 4 to 6 minutes
C. from 6 to 14 minutes
D. from 14 to 16 minutes
is 5 miles due East of her starting point but she had to
drive 7 miles total since roads don’t go due East.
Which of the following best describes the
displacement Jill experienced?
A. Jill was displaced 7miles.
B. Jill was displaced 5 miles.
C. Jill was displaced 7 miles East.
D. Jill was displaced 5 miles East.
The trip takes approximately 2 hours and covers
approximately 1900 km. Which of the following best
describes the average speed and direction of the jet’s
flight?
A. 475 km/h southwest
B. 950 km/h southwest
C. 1900 km/h southwest
D. 3800 km/h southwest
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: D
Average speed is found by dividing distance by
time. 1900km / 2hr = 950km/hr. The direction
indicated in the picture is southwest.
Displacement is how far you are from your
starting point, regardless of how you got there.
Since Jill ended up 5 miles East of where she
started, that is her displacement.
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Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: A
CORRECT ANSWER: C
Average speed is total distance over total time.
The ball went 12 meters in 6 seconds, 12m/6s =
2 m/s. (explanations could also involve slope
of the line)
If the beetle is not moving, its position is not
changing, this would be the horizontally flat
section of the graph from 6 – 14 minutes.
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9.!! An object is traveling in a straight line. The graph
below shows the object’s velocity over time.
10.!!The graph below represents the motion of a car as it
moves along a straight road for 20 s.
1.4 Interpret and apply Newton’s three laws of motion.
11.!!You are driving along at a constant 20 miles per
12.!!The illustration below shows a 2-ton elephant
hour. Then you slam on the brakes. What happens
to your books that are on the passenger side seat?
A. The books stay on the seat.
B. The books continue moving forward and fall on
the floor.
C. The books fly up in the air and end up in the back
seat.
D. The books slide across the seat and hit the door.
Which line segment shows the object traveling with a
constant, positive acceleration?
A. segment W
B. segment X
C. segment Y
D. segment Z
Which statement best describes the motion of the
car over the 20 s?
A. The car is initially at rest and then backs up for
10 s.
B. The car moves 20 m in the first 10 s and then
stops.
C. The car travels at a constant speed for 10 s and
then decelerates until it stops.
D. The car travels at a constant speed of 20 m/s and
then decelerates until it is traveling at a constant
speed of 15 m/s.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: C
CORRECT ANSWER: C
Constant, positive acceleration is shown by a
constant, positive slope on a velocity vs time
graph.
This is a speed vs time graph. Read the value of
the velocity directly off the graph. The speed is
constant for the first 10 seconds (= 20 m/s) and
then it begins slow down until at t = 20s the
speed is 0 m/s (it stops).
!
balancing on a tree stump.
Which of the following statements must be
accurate?
A. The weight of the tree stump is greater than 2
tons.
B. A 4-ton force on the ground spreads out in all
directions.
C. The tree stump is exerting a 2-ton force upward
on the elephant.
D. The downward force on the ground under the tree
stump is 4 tons.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: C
Newton’s First Law of motion tells us that
books (which have mass) will continue to move
in a straight line at a constant speed until an
unbalanced force acts on them. Since there
isn’t an unbalanced force on the books, they
keep moving forward.
Because the elephant is at rest, the net force
acting on him must be zero. There is a 2-ton
force pulling him down that is balanced by a 2ton force from the stump pushing back up on
him.
!
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13.!!A mosquito flies into the oncoming windshield of a
large truck. The truck exerts a force of 10 Newtons
on the mosquito. What can we say about the force
that the mosquito exerts on the truck?
A. The mosquito exerts a force much less than 10N
on the truck.
B. The mosquito exerts a force exactly equal to 10N
on the truck.
C. The mosquito exerts a force much greater than
10N on the truck.
D. Not enough information is provided.
14.!!You punch a wall. Afterwards your hand hurts so
you know that the wall must have exerted a force on
your hand. The wall didn’t move or react. Did you
exert a force on the wall?
A. Yes, I exerted a force on the wall. Forces always
occur in pairs. The force that I exerted is equal in
magnitude and opposite in direction to the force
that the wall exerted on me.
B. Yes, I exerted a force on the wall. Forces always
occur in pairs. The force that I exerted on the
wall is much smaller than the force that the wall
exerted on me which is why the wall doesn’t
move.
C. No, I didn’t exert a force on the wall. If I had
exerted a force on the wall, the wall would have
moved.
D. No, I didn’t exert a force on the wall. You can’t
exert forces on solid objects like walls.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: A
Forces always come in pairs that are equal in
magnitude and opposite in direction (Newton’s
rd
3 Law.) Because the truck exerts a force of
10N on the mosquito, the mosquito must exert
a 10N force on the truck.
This is simply due to Newton’s 3 Law.
Although you couldn’t see the wall move or
react, it did just a little bit but its mass was so
big it was difficult to notice.
15. ! A ball has a mass of 0.1 kg and an initial velocity of
20 m/s. The ball is given an acceleration of 30
m/s2 for 5 s.
16.!!A tiger football player with a mass of 100kg is
running down the field. A player from the other
football team exerts a force of 500N to the left on
the tiger and the tiger experiences a friction force of
100N to the right during the tackle. What is the
magnitude of the tiger’s acceleration?
A. 1m/s2
B. 4m/s2
C. 5m/s2
D. 6m/s2
What is the net force on the ball during
acceleration?
A. 0N
B. 3N
C. 10N
D. 15N
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: B
F=?
m = 0.1 kg
2
a = is 30m/s
Fnet = - 500 N + 100 N
Fnet = -400 N, (to the left)
a = Fnet / m
a = -400 N / 100 kg
2
a = -4 m/s
According to Newton’s 2
F=ma
F = (0.1)(30)
F=3N
nd
Law:
rd
1.5 Use a free-body force diagram to show forces acting on a system consisting of a pair of
interacting objects. For a diagram with only co-linear forces, determine the net force acting on a
system and between the objects.
17.!!The forces acting on a skateboarder moving at a
18.!!An elastic cord made for bungee jumping is being
constant velocity along a sidewalk are shown in the
figure below.
Which of the following is the net force on the
skateboarder?
A. 0 N
B. 70 N
C. 670 N
D. 1270 N
tested. A weight of 800 N is attached to one end of
the bungee cord. Then the weight is released from a
tall tower and it moves downward. When the elastic
cord is fully extended, it exerts an opposing force of
900 N on the weight. Which of the following is an
accurate free-body diagram for this situation?
A.!!
!
800N!
!
!
!
!
!
800!N!
!
!
C.!!
!
!
!
!
800N!
!
! 900!N!
!
!
!
!
!
B.!!
800N!
900!N!
D.!!
900N!
800!N!
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: A
CORRECT ANSWER: D
Sum of forces in X direction: +35 N – 35 N = 0 N
Sum of forces in Y direction = Normal force + Weight
+600 N – 600 N = 0 N
Weight is a downward force of 800 N
Elastic force is upward and equals 900 N
2
The tigers acceleration is 4 m/s left.
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1.6 Distinguish qualitatively between static and kinetic friction, and describe their effects on the
motion of objects.
19. You place a block on a slide at the park. The block
20.!!An object that you are pushing with a force of 10
stays at rest on the slide. What force keeps the block
in place?
A. weight
B. static friction
C. kinetic friction
D. normal force
student is swinging the ball in a horizontal circle.
constant speed in a circular path.
Newtons is sliding at a constant velocity. What do
you know about the friction acting on the object?
A. 10 N of kinetic friction are acting on the object.
B. 10 N of static friction are acting on the object.
C. Less than 10N of static friction are acting.
D. Less than 10N of kinetic friction are acting.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: A
Friction opposes the force of gravity. Since the
block is not moving the force that prevents it
from moving is Static Friction
Since the block is moving at a constant velocity
the acceleration is zero. Since the acceleration
is zero the net force is zero, this implies there is
a force opposing the applied force of 10 N. This
force is friction. It is kinetic friction because
the block is sliding.
1.7 Describe Newton’s law of universal gravitation in terms of the attraction between two
objects, their masses, and the distance between them.
21.!!One 7.0 kg bowling ball is lifted to a storage shelf
22.!!On Earth, Johanna weighs 100 lbs. She calculated
1.0 m above the floor. A second 7.0 kg ball is lifted
to a storage shelf 2.0 m above the floor.
Which of the following best explains why the
measured force of gravity on each ball is nearly
identical?
A. The final potential energy of each ball increased.
B. The amount of work required to lift each ball is
identical.
C. The distance of each ball from Earth’s center of
mass is almost identical.
D. The gravitational force of each ball on the other
cancels out the force of Earth’s gravity.
1.8 Describe conceptually the forces involved in circular motion.
23.!!The diagram below shows a ball tied to a string. A
24.!!The diagram below shows an object traveling at a
what her weight would be at several other locations
in the solar system. The results are shown in the
table below.
If the student releases the string, in which direction
will the ball travel?
A. W
B. X
C. Y
D. Z
Which labeled arrow represents the centripetal force
acting on the object?
A. W
B. X
C. Y
D. Z
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: D
The arrow labeled X is tangent to the circle at
the point the ball is released. This is the same
as the direction of the ball’s velocity at the
moment of release. When released, no
horizontal force will act to change the velocity
of the ball, so the ball will continue on that
straight path in accord with the principle of
inertia.
The arrow labeled Z points inward, toward the
center of the ball’s circular path. The centripetal
force (required to change the ball’s velocity)
points toward the center of the circular path, so
arrow Z matches the centripetal force.
Which of the following statements is best supported
by the information in the table?
A. Venus has more gravitational force than Earth.
B. Mars has less gravitational force than the Moon.
C. Earth has four times the gravitational force of
Mars.
D. Jupiter has more than twice the gravitational
force of Earth.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: C
CORRECT ANSWER: D
The force of gravity between two objects (in
this case, Earth and a bowling ball) depends
upon their masses, and the distance separating
their centers of mass. In this case, lifting a ball
1 m does not significantly change the distance
between the ball and Earth’s center
(>6,000,000 m).
According to the table, Johanna’s weight on
Jupiter (260 lbs.) would be over twice that of
her weight on Earth (100 lbs.). Weight is the
same as gravitational force, so the gravitational
force on her at Jupiter would be over twice that
of Earth, in agreement with answer D.
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2. Conservation of Energy and Momentum
Central Concept: The laws of conservation of energy and momentum provide alternate
approaches to predict and describe the movement of objects.
2.1 Interpret and provide examples that illustrate the law of conservation of energy.
25.!! The diagrams below show a man swinging a golf
26.!!A ball is thrown straight upward. The ball’s initial
club.
speed is 30 m/s and its mass is 0.05 kg, resulting in
an initial kinetic energy of 22.5 J. If the initial
potential energy of the ball is 10 J and there is no
frictional force, what would be the ball’s total
energy while it is moving?
A. 0.0 J
B. 10.0 J
C. 22.5 J
D. 32.5 J
The kinetic energy of the golf club at point Y is
more than the potential energy of the club at point
X. Which of the following statements best explains
why this fact does not violate the law of
conservation of energy?
A. Gravity is constant from point X to point Y.
B. Air resistance is greater at point Y than at point
X.
C. Acceleration due to gravity is greater at point Y
than at point X.
D. Energy is added by the man to the golf club from
point X to point Y.
2.2 Interpret and provide examples of how energy can be converted from gravitational potential
energy to kinetic energy and vice versa.
27.!!Which one of the following objects has mechanical
28.!!An astronaut drops a 1.0 kg object and a 5.0 kg
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.
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.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: B
CORRECT ANSWER: D
For the total mechanical energy (KE + GPE) to be
remain constant, there should be no friction involved
in the system. In addition, there should not be work
done by an external force to the system. The apple in
free-fall qualifies for this description because the
only force acting on this system is gravity, which
accelerates the apple as it falls. The apple will then
lose GPE but gains KE to make the total mechanical
energy remain constant.
The total mechanical energy has to be
conserved in a system without friction. When
the astronaut drops the two masses half way
(for 1m instead of the total distance of 2m),
each masses would have lost half of the GPE
during the drop. As a result, they would gain
about half of their maximum KE so their total
energy stays the same.
2.3 Describe both qualitatively and quantitatively how work can be expressed as a change in
mechanical energy.
29.!!An archer pulls back the bowstring to prepare to
30.!!Work is performed on an object by raising it 2 m
shoot an arrow as shown below.
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: D
CORRECT ANSWER: D
We are told the KE at the bottom of the swing is
more than the PE of the club at the top of its
swing. According to the conservation of
energy, the total energy of the club must be
constant unless energy is added to the club.
The extra KE, above and beyond the initial PE,
comes from the man, who is able to do work
and thus add energy to the club.
[Note that the other choices can be argued to
be true statements in themselves, but they do
not explain the increase in the club’s energy.]
Since Total Energy = KE + GPE when friction is
not present. In this situation, the friction is not
present so the total energy does not change
throughout the ball’s journey.
Initial KE = 22.5 J
Initial PE = 10J
Total Energy Initial = 22.5 + 10 = 32.5J
(No Elastic Potential Energy in this situation)
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
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: A
CORRECT ANSWER: C
F = 40N
d = 0.2m
When an object is being raised above a surface,
its gravitational potential energy increases as
the height increases. Since mechanical energy
include gravitational potential energy, the
object’s total mechanical energy has increased.
W=Fd
W = (40N) ( 0.2m)
W = 8 N-m or 8 J
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above the floor. Which of the following types of
energy must change in this situation?
A. chemical energy
B. magnetic energy
C. mechanical energy
D. thermal energy
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2.4 Describe both qualitatively and quantitatively the concept of power as work done per unit
time.
31.!!At a weightlifting competition, two competitors
32.!!The table below provides information about the
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.
daily use of some typical appliances.
Appliance
Power
Rating
(W)
Duration
of Use
(hr)
dishwasher
1500
0.5
air conditioner
1000
1.0
laptop computer
50
12.0
clock radio
1
24.0
Which appliance uses the most energy?
A. dishwasher
B. air conditioner
C. laptop computer
D. clock radio
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: C
CORRECT ANSWER: B
Since the competitors lifted the same weight to
the same height, their work was the same
(work=force • distance). However, since the
second competitor did it faster, he or she had
greater power. Power = work/time. A smaller
amount of time (denominator) means a greater
amount of power.
Use the equation
!
!!!!!!!!!!! =
!
!=!∗!
!
(remember that work is a form of energy). The
air conditioner uses 1000 W for 1 hr, and the
product of those 2 numbers is the largest value
in the table.
2.5 Provide and interpret examples showing that linear momentum is the product of mass and
velocity, and is always conserved (law of conservation of momentum). Calculate the momentum
of an object.
33.!!You are at an ice skating rink and are gliding
34.!!An object with a mass of 3 kg has a momentum of
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.
Open Response: Please complete on a separate sheet of paper.
• 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.
1.!! A!person!is!driving!his!car!to!a!store.!The!store!is!800!m!north!of!the!person’s!starting!
point.!!After!traveling!north!1200!m,!the!person!realizes!that!he!has!passed!the!store.!He!
turns!the!car!around!and!drives!back!to!the!store.!Altogether,!the!trip!to!the!store!takes!
100s.!
!
a.! Determine!the!distance!traveled!by!the!car!on!this!trip!to!the!store.!Show!your!
calculations!and!include!units!in!your!answer.!
!
b.! Calculate!the!average!speed!of!the!car!on!this!trip.!Show!your!calculations!and!
include!units!in!your!answer.!
!
c.! Determine!the!displacement!of!the!car!for!this!trip.!Include!units!in!your!answer.!
!
d.! Calculate!the!average!velocity!of!the!car!upon!arrival!at!the!store.!Show!your!
calculations!and!include!units!in!your!answer.
Answer:
1a.)
Distance is the total amount of meter traveled. This person drove 1200 m
first and then he or she had to go back 400 m:
d = 1200 m + 400 m
d = 1600 m
1b.)
s=?
d = 1600 m
t = 100 s
75 kg • m/s. What is its velocity?
A. 0.4m/s
B. 7.1m/s
C. 25m/s
D. 72m/s
1c.)
s=
d
t
s=
1600 m
100 s
s = 16 m/s
Displacement is the difference between the starting and ending points.
Δx = 1200 m – 400 m
Answer and Explanation:
Answer and Explanation:
CORRECT ANSWER: C
CORRECT ANSWER: C
When you collide with your friend, momentum
is conserved. The mass of you and your friend
is more than the mass of you by yourself.
Since momentum is m • v, then if the final
mass is increased, the final velocity must
decrease.
Momentum = mass • velocity.
75 kg • m/s = 3 kg • velocity (divide both sides
by 3 kg)
25 m/s = velocity
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Δx = 800 m North
1d.)
v=?
Δx = 800 m North
t = 100 s
v=
Δx
t
v=
800 m North
100 s
v = 8 m/s North
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2. A!book!is!on!a!table.!A!student!pushes!it!for!a!short!time.!Initially!the!book!moves,!but!
then!it!comes!to!a!complete!stop.!
!
a.! Identify!the!forces!acting!on!the!book!before!it!is!pushed.!You!may!include!a!labeled!
diagram!in!your!answer.!
!
b.! Explain!why!the!book!moves!and!then!comes!to!a!complete!stop.!Use!the!laws!of!
physics!in!your!answer.!
!
c.! The!student!wants!the!book!to!move!at!a!constant!speed!in!one!direction.!Describe!
the!physical!conditions!needed!for!this!to!occur.!
Answer: (SAMPLE 4 point response)
3. Students!in!a!physics!class!are!studying!the!energy!changes!that!objects!experience!as!
they!fall.!The!students!plan!to!drop!a!metal!sphere!with!a!mass!of!0.05!kg!from!a!height!
of!20.0m!onto!a!bed!of!sand.!They!predicted!the!results!shown!in!the!table!below.!
Time!(s)!
Speed!(m/s)!
Distance!Fallen!(m)! Height!(m)!
0.0!
0!
0.0!
20.0!
0.5!
5!
1.3!
18.7!
1.0!
10!
5.0!
15.0!
1.5!
15!
11.3!
8.7!
2.0!
20!
20.0!
0.0!
a.! Using!the!predicted!values!in!the!table,!calculate!the!kinetic!energy!of!the!sphere!just!
before!it!contacts!the!sand!at!2.0!s.!Show!your!calculations!and!include!units!in!your!
answer.!
b.! Using!the!predicted!values!in!the!table,!calculate!the!gravitational!potential!energy!
at!0.0!s.!!Show!your!calculations!and!include!units!in!your!answer.!
c.! Describe!what!happens!to!the!gravitational!potential!energy!and!the!kinetic!energy!
as!the!sphere!falls.!!
d.! After!the!sphere!is!dropped,!the!students!find!that!the!results!of!the!experiment!are!
slightly!different!than!they!predicted.!Assuming!the!students!performed!their!
calculations!correctly,!explain!one!possible!reason!for!the!difference!they!observed.!
!
Answer:
3a.) At time = 2 s the speed is 20 m/s
v = 20 m/s
m = 0.05 kg
1
1
KE = m v 2
KE = (0.05 kg)(20 m/s)2
KE = ?
KE = 10 J
2
2
!
3b.) At time = 0 s the height is 20 m
h = 20 m
m = 0.05 kg
PE = m g h
PE = (0.05 kg) (10 m/s2 )(20 m)
PE = ?
PE = 10 J
!
3c.) Gravitational Potential Energy is directly proportional to how high the sphere is
above the ground. As the sphere falls down, the height decreases, so does the
Gravitational PE. We know that mechanical energy is conserved when gravity is the
only force doing work. In this case, we assume gravity is the only force acting on
the sphere, so mechanical energy is conserved. This means that PE + KE stays the
same. If the gravitational PE is decreasing, then KE must be increasing.
PE is being transformed into KE as the sphere falls down.
!
3d.)
ATTENTION: THESE ANSWER KEYS ARE INTENDED TO BE USED IN SCHOOL AND UNDER
TEACHER SUPERVISION. (No pictures, photocopying, or posting them online, please)!
Assuming that gravity is the only force acting on the sphere as it falls is not
completely correct. There is a small amount of air resistance acting on it. This
means that some of the Gravitational PE the sphere had at the very top is not going
to be converted into KE, instead it will be converted into heat, due to the rubbing
against air molecules. This means that the predicted speeds of the sphere will be
slightly faster than the measured speeds of the sphere.
ATTENTION: THESE ANSWER KEYS ARE INTENDED TO BE USED IN SCHOOL AND UNDER
TEACHER SUPERVISION. (No pictures, photocopying, or posting them online, please)!