Physics 120 Practice Midterm 2

1) You need to make a sharp turn on a flat 4) A piano mover raises a 100 kg piano at a
road, making a radius of curvature of 15 constant rate using a frictionless pulley
meters. How does the required force of system, as shown below. With roughly what
static friction between your tires compare if force is the mover pulling down on the
you make the turn at 30 mph vs. 60 mph? rope?
A) The force of friction needs to be twice
as large.
B) the force of friction is the same for
both speeds since the radius of
curvature is the same.
C) The force of friction needs to be four
times as large.
D) None of the above

2) A person ties a rock to a string and whirls it

around in a circle such that sometimes the
rock is going straight upward and
sometimes the rock is going straight down
(the plane of the circle is perpendicular to
the horizon). She whirls the rock at the
minimum speed (constant in time) such that
the string is always taunt (no sag). If she
were to use a longer string, she would have A) 500 N
to whirl the rock at a B) 2000 N
A) lower velocity. C) 1000 N
B) the same velocity. D) 250 N
C) higher velocity. E) Depends on the velocity!

3) A large truck collides head-on with a 5) A star is moving towards the Earth with a
cyclist. During the collision speed of 0.9 c (90% the speed of light). It
A) the truck exerts the same amount of emits light, which moves away from the star
force on the cyclist as the cyclist at the speed of light. Relative to us on Earth,
exerts on the truck. the speed of the light moving towards us
B) the truck exerts a greater amount of (emitted by the star) is
force on the cyclist than the cyclist A) 1.2 c B) 0.98 c
exerts on the truck. C) 1.9 c D) c
C) the truck exerts a force on the cyclist,
but the cyclist exerts no force onto the 6) Two friends are standing on opposite ends
truck. of a canoe. The canoe is initially at rest with
D) the truck exerts a smaller amount of respect to the lake. The person on the right
force on the cyclist than the cyclist throws a very massive ball to the left, and
exerts on the truck. the person on the left catches it. After the
ball is caught, the canoe is (ignore friction
between the canoe and the water)
A) moving to the left.
B) moving to the right.
C) stationary

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7) A spring-loaded dart gun is used to shoot a 10) You are standing on a skateboard, initially
dart straight up into the air, and the dart at rest. A friend throws a very heavy ball
reaches a maximum height of 24 meters. towards you. You can either catch the object
The same dart is shot up a second time from or deflect the object back towards your
the same gun, but this time the spring is friend (such that it moves away from you
compressed only half as far (compared to with the same speed as it was originally
the first shot). How far up does the dart go thrown). What should you do in order to
this time (neglect friction and assume the minimize your speed on the skateboard?
spring obeys Hooke’s law)? A) catch the ball
A) three meters B) 12 meters B) Your final speed on the skateboard
C) 48 meters D) six meters will be the same regardless whether
you catch the ball or deflect the ball.
8) A person ties a rock to a string and whirls it C) deflect the ball
around in a circle such that sometimes the
rock is going straight upward and 11) Joe and Bill throw identical balls vertically
sometimes the rock is going straight down upward. Joe throws his ball with an initial
(the plane of the circle is perpendicular to speed twice as high as Bill. The maximum
the horizon). She whirls the rock at the height of Joe’s ball will be
minimum speed (constant in time) such that A) two times that of Bill’s ball.
the string is always taunt (no sag). When is B) four times that of Bill’s ball.
the tension the highest? C) roughly 1.3 times that of Bill’s ball.
A) The tension is constant as the rock D) equal to that of Bill’s ball.
moves around in a circle. E) eight times that of Bill’s ball.
B) It is highest when the rock is at the
lowest elevation.
C) It is highest when the rock is at the 12) An astronomer on earth sees a meteor hit
highest elevation. the Moon at exactly the same time as he sees
a solar flare erupt on the sun. The solar
9) Bill and Susan are both standing on flare and the meteor impact are
identical skateboards (with really good ball simultaneous events in this astronomer’s
bearings), initially at rest. Bill weighs three reference frame.
times as much as Susan. Bill pushes
horizontally on Susan’s back, causing Susan 13) In their common rest frame, two stars are
to start moving away from Bill. 90.0 ly apart. If they appear to be 42.1 ly
Immediately after Bill stops pushing, apart to a spaceship, how fast is the
A) Susan and Bill are moving away from spaceship moving? Express your answer in
each other, and Susan’s speed is three terms of c.
times less than that of Bill. A) 0.823 c B) 0.884 c
B) Susan and Bill are moving away from C) 0.845 c D) 0.865 c
each other, with equal speeds.
C) Susan and Bill are moving away from
each other, and Susan’s speed is three
times that of Bill.
D) Susan is moving away from Bill, and
Bill is stationary.

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14) The figure shows a 3.0 kg ball tied to the 17) An object attached to a spring is pulled
end of a 50 cm long string being swung in a across a frictionless surface. If the spring
circle in a vertical plane at constant speed. constant is 45 N/m and the spring is
The center of the circle is h = 230 cm above stretched by 0.88 m when the object is
the floor. The ball is swung at the minimum accelerating at 2.0 m/s2, what is the mass of
speed necessary to make it over the top the object?
without the string going slack. If the string A) 20 kg B) 26 kg
is released at the instant the ball is at the top C) 22 kg D) 17 kg
of the loop, how far to the right of the center
of the circle does the ball hit the ground? 18) A new roller coaster contains a
loop-the-loop in which the car and rider
are completely upside down. If the radius of
the loop is 12.6 m, with what minimum
speed must the car traverse the loop so that
the rider does not fall out while upside
down at the top? Assume the rider is not
strapped to the car.
A) 14.9 m/s B) 11.1 m/s
C) 12.2 m/s D) 10.1 m/s

19) A 18.7 kg child must exert a 350.0 N radial

force in order to remain on her mount on
the outer edge of a merry-go-round.
Assuming that the child is moving at
8.00 m/s, what is the diameter of the
merry-go-round?
A) 6.84 m B) 3.42 m
C) 7.11 m D) 3.56 m

20) If you swing a bucket of water fast enough

A) 170 cm B) 130 cm
in a vertical circle, at the highest point the
C) 150 cm D) 0.0 cm
water does not spill out because an outward
force balances the pull of gravity on the
15) A 100 g ball of clay is thrown horizontally
water.
with a speed of 90.0 m/s toward a 900 g
block resting on a frictionless surface. It hits
21) You pull on a crate with a rope. If the crate
the block and sticks. The clay exerts a
moves, the rope’s pull on the crate must
constant force on the block during the 10.0
have been larger than the crate’s pull on the
ms it takes the clay to come to rest relative
rope, but if the crate does not move, both of
to the block. After 10.0 ms, the block and
these pulls must have been equal.
the clay are sliding along the surface as a
single system. What is the force of the clay
22) In a ballistics test, a bullet traveling
on the block during the collision?
horizontally strikes and embeds itself in a
A) 4050 N B) 8100 N
large block which is free to move. During
C) 810 N D) 900 N
this process, the bullet transfers all of its
momentum to the block.
16) A child swings a 0.37 kg ball in a circle on a
string that is 1.3 m long. If the ball makes
23) As a box slides along a rough horizontal
1.2 rev/s, what is the magnitude of the
surface and comes to rest, its kinetic energy
ball’s angular momentum?
is transformed into potential energy.
A) 0.8 kg·m2/s B) 5.2 kg·m2/s
C) 3.7 kg·m2/s D) 4.7 kg·m2/s

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Answer Key
Testname: MIDTERM2_PRACTICE

1) C
2) C
3) A
4) A
5) D
6) C
7) D
8) B
9) C
10) A
11) B
12) FALSE
13) B
14) A
15) C
16) D
17) A
18) B
19) A
20) FALSE
21) FALSE
22) FALSE
23) FALSE