Download Exam 2 Physics 125 Fall 2008 Name:

Exam 2 Physics 125 Fall 2008
Name:
Time Allowed: 2 hours
(g=10 m/s2 unless stated otherwise, G=6.67×10-11 N-m2/kg2, 1 lb= 4.45N)
PART I (60 points)
Answer the following 15 questions, worth 4 points each
Numerical answers require supporting calculations
1. Describe in your own words the meaning of inertia
2. The diagram below is a plot of acceleration against the net force on an object. What is the mass of the
object?
0.8
a(m/s2)
0.6
0.4
0.2
5
10
15
20
25
F(N)
3. The weight of a person on the moon is 28 lb. The acceleration due to gravity of the moon is 1/6 that on
earth. What is the mass of the person on earth?
(a)
(b)
(c)
(d)
125 kg
75 kg
45 kg
13 kg
4. A 25-kg child and 75-kg adult standing together on skates slowly push each other away. As a result, the
child accelerates at 1.2 m/s2. What must be the acceleration of the adult?
(a)
(b)
(c)
(d)
3.6 m/s2
1.2 m/s2
0.8 m/s2
0.4 m/s2
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5. Immediately after a football on the ground is kicked, it acquires a velocity whose magnitude is 25 m/s
and whose direction is at 65° with the horizontal. Neglecting air resistance, find how long the ball stays
in the air.
(a)
(b)
(c)
(d)
2.3s
3.4s
4.5s
5.8s
6. What is the magnitude and direction of the acceleration of the football in the previous problem, if any,
when it reaches maximum height?
7. A 25-kg child standing on skates in a skating ring is pushed by a constant force of 8.0 N. Starting from
rest, what is the final speed of the child after 3.0 s if friction can be neglected?
(a)
(b)
(c)
(d)
0.96 m/s
1.23 m/s
2.92 m/s
3.44 m/s
8. The diagram below shows a crate on a flatbed truck which is slowing down. The crate does not slip.
Draw a free body diagram for the crate that identifies all forces acting on it.
9. The diagram below shows a shopping cart being pushed by a force of 24 N in the direction shown. The
weight of the cart is 330 N. Find the normal force on the cart.
54°
(a)
(b)
(c)
(d)
354N
349N
311N
305N
2
10. During vertical blast-off, a rocket attains an upward acceleration of 3g. What is the apparent weight of a
216 lb astronaut sitting in the rocket?
(a)
(b)
(c)
(d)
72 lb
216 lb
648 lb
864 lb
11. Blowing wind exerts a force of 2,500N on the sail of a sail boat weighing 5,000N. The boat is observed
to travel with an acceleration of 1.5 m/s2 downwind. Find the force of resistance due to the water.
(a)
(b)
(c)
(d)
1250N
1500N
1750N
2000N
12. The fact that all objects fall with the same acceleration on the surface of the earth is explained by
(a)
(b)
(c)
(d)
Newton’s law of universal gravitation alone
Newton’s law of universal gravitation and Newton’s first law of motion
Newton’s law of universal gravitation and Newton’s second law of motion
Newton’s law of universal gravitation and Newton’s third law of motion
13. The diagram shows three blocks on a frictionless table connected together with two identical strings,
which can be subject to a maximum tension of 50.0N before breaking. The masses of the blocks are
indicated, and the assembly is pulled by a horizontal force P. What is the maximum acceleration the
blocks can experience without breaking either one of the strings?
5kg
(a)
(b)
(c)
(d)
4kg
3kg
P
10 m/s2
5.6 m/s2
4.2 m/s2
3.8 m/s2
3
14. A car rolls down a slope inclined at 25° with the horizontal. What is the normal force on the car if its
mass is 1200 kg?
(a) 507 N
(b) 5,070 N
(c) 1,088 N
(d) 10,880 N
15. A 5.0 kg block on a smooth table is pulled by two mutually perpendicular forces of 4.8N and 3.5N
respectively. Find the magnitude of the acceleration of the block.
(a)
(b)
(c)
(d)
1.7 m/s2
1.2 m/s2
0.96 m/s2
0.26 m/s2
PART II (40 points)
16. An astronaut visits a planet where the acceleration due to gravity is 40 m/s2.
(a) (5 points) He releases a rock at zero speed on top of a 200m cliff. How fast is the rock traveling
right before hitting ground?
(b) ( 5 points) He then throws another rock horizontally out with a speed of 100 m/s. What is the
speed of this rock right before hitting ground?
(c) ( 5 points) He knows that the radius of the planet is 1,600 km. Can he determine the mass of the
planet? Give the number if your answer is yes.
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17. The diagram shows a 5.0 kg block on a frictionless table connected by a string passing over a pulley to
a hanging block of unknown mass, which is initially held at 30 cm above the floor.
(a) ( 2 points) When released from rest, the hanging block acquires a speed of 2.2 m/s after before
landing on the floor. What is the travel time?
(b) ( 4 points) What is the tension on the string during the travel ?
(c) ( 4 points) What is the mass of the hanging block?
5
18. Consider a suitcase on a horizontal conveyer belt such as can be seen in the luggage pick up area in an
airport. The weight of the suitcase is 125 N. The coefficients of static and kinetic friction of the conveyer belt
are 0.2 and 0.15 respectively.
.
(a) (5 points) What is the friction force on the suitcase when the belt is moving at a constant velocity of 0.6
m/s ?
(b) ( 5 points) What is the friction force on the suitcase if the belt moves with acceleration 1.4 m/s2 and the
suitcase does not slip/
(c) ( 5 points) Explain what happens if the acceleration of the belt is 2.0 m/s2
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