Second Examination
Physics 103
21 October 1980

#1.A box of mass 10.0 kg is at rest on a horizontal floor, and the coefficient of static friction is µs = 0.20. a force, F, acts downward on the box at an angle of 30o to the horizontal, as sketched.  The value (in Newtons) which the magnitude of F must exceed if the box is to be set in motion on the floor is most nearly equal to:

Answer:
a) 98  b) 20 c) 26 d) 2 e) 490

#2.  The braking force required to stop an automobile weighing 3200 lb on a level road (µk = 0.3), assuming the drive "jams" on the brakes and decelerates at a =  -10 ft/sec2, is
Answer:

a) 960 lb b) - 40 lb       c)1960 lb    d)  - 960 lb     e) - 9600 lb

#3.  A block of mass m is held at rest on a frictionless inclined plane by means of a cord, as shown in the sketch.  The cord is stretched parallel to the plane under tension T, and the plane is inclined at an angle  q above the horizontal.  If N is the normal force exerted by the plane, the correct free-body-diagram for the block is

Answer:
a) b) c)
d) e)

#4.   Mass mA is connected to mass mB by a massless cord hung over a massless, frictionless pulley, as shown.  The kinetic coefficient of friction between mA and the table is µ.  The mass mB accelerates downward.  The positive acceleration of mB is equal to

Answer:

a)  g    b)  [mB - µ mA] / [mA + mB] g    c)  µ mA/ [mA + mB] g    d)   µ g     e) Ö (2 µ) g

#5.  A conical pendulum rotates as shown with the string making an angle of 37o with the vertical support.  If the distance from the ball to the support wall is 0.5 meter, the linear speed of the ball is about

Answer:

a) 3.7 m/sec   b) 2.9 m/sec   c) 7.6 m/sec   d) 1.9 m/sec   e) 0 m/sec

#6.  A railroad flatcar loaded with crates has a coefficient of static friction µs between the crates and the floor.  If the train is moving with speed V, the shortest distance in which the train can be stopped without letting the crates slide is equal to
Answer:

a) V2 / (2µsg)  b) µsV2 / Ö(2sg)    c) sV2 / g    d) (1- µs)V2/g    e) 2V2 / (1 -µsg)

#7. Two blocks are in contact with a frictionless, horizontal table.  If m1 = 3.0 kg and m2 = 5.0 kg, and a horizontal force of 2.0 Nt acts on m1 (see sketch), the magnitude of the force of contact between the two blocks is most nearly equal to

Answer:
a) 0.33 Nt   b) 0.25 Nt    c) 0.50 Nt     d) 1.00 Nt       e) 1.25 Nt

#8. A block of mass M = 15 kg starts with a velocity of v0 = 1 m/sec on a 30o incline at a distance of 1 meter above the ground.  It slides down the rough incline and has a velocity vf = 3 m/sec when it reaches the ground.  What is the coefficient of kinetic friction?

Answer:
a) 0.59       b) 0.30       c) 1.0            d) 0.34       e) 0.68

#9. A ball dropped from height h1 with zero initial speed hits the sidewalk and bounces straight back up to a lesser height h2.  The total work done on the ball by air resistance and the sidewalk during this process is
Note: The ball does not spin at any time.

Answer:
a) mg (h1 + h2   b) zero        c) mg h2      d) mg (h1 - h2)       e) mg (h2 - h1

#10. The internal energy generated by a hockey puck (m = 0.5 kg) sliding on level ice (µk = 0.10) for 70 meters is approximately
Answer:
a) 34.3 Joules    b) 3.5 Joules   c) 0 Joules    d) - 3.5 Joules    e) - 34.3 Joules

#11. A block of mass 10 kg compresses a light spring (k = 1000 Nt/m) by a distance of 0.25 m from its equilibrium position.  Neglecting friction, the velocity of the block when the spring reaches its equilibrium position is most nearly equal to

Answer:
a) 2.5 m/sec    b) 26.3 m/sec    c)  31.3 m/sec    d) 250 m/sec    e) 0.6 m/sec

#12. The block in Problem #11 then slides on a rough surface (µk = 0.15).  The distance, d, it travels before coming to rest is approximately
Answer
a) 4.1 m     b) 21 m        c) 10.5 m       d) 1.1 m        e) 2.1 m

#13. A roller coaster starts approximately from rest at the top of a hill 90 feet high.  It goes on a smooth track through a loop of diameter 30 ft.  At the top of the loop, the roller coaster's velocity ( in ft/sec) is most nearly equal to

Answer

a) 3840 ft/sec    b)  62 ft/sec     c)  31 ft/sec     d) 0 ft/sec   e) 124 ft/sec

14. A string has a length of 1.5 m.  A ball (mass 2.0 kg) is attached at one end, and the other end is fixed to a frictionless pivot.  The ball is held so that it stretches the string horizontally, and it is then released.  (See sketch)  When it reaches the lowest point on its swing, its speed (in m/sec) is most nearly equal to

Answer:
a) 14.7      b) 3.8    c) 6.5       d) 5.4       e) 3.0

#15. A man lifts ten 7 kg boxes per minute to a conveyer belt which is 21.5 meters off the ground.  How much power does he exert in doing this?
Answer:
a) 1.7 Watts     b)17 Watts      c) 101.9 Watts     d) 10.3 Watts      e) 0.17 Watts