Physics 201/211: Test 2

9:30AM Thursday, October 10, 1996

NAME (printed):

Student Number (SSN):

Recitation Section Number :

INSTRUCTIONS:

• Wait for the oral instruction to begin.
• Show all your work in order to qualify for partial credit. Write legibly!
• Assist the graders by placing a box around your final answer.
• You may only use your calculator for simple arithmetic operations.

You may find the following data useful:

, downward.

Note the formula sheet appended to the end of the test.

This test is worth 50 points. Good Luck!

: Multiple Choice. Circle the most suitable answer for each of the questions enumerated below. There is no partial credit for this section. Each question is worth 4 points.

1. A ball is thrown with some unspecified and nonzero initial velocity from the top of a cliff. From the angle with which it is thrown we know that the vertical component of its initial velocity is upward. At what point in its subsequent trajectory does it possess minimum speed? (N.B. The moment at which it is thrown is t=0.)

a)

At t=0, so that it doesn't have a chance to fall.

b)

A moment before it hits the ground.

c)

At  , where we take the vertical direction as y.

d)

At  , where we take the vertical direction as y.

2. A falling parachutist floats slowly to the ground at a constant velocity after opening her parachute. If the combined mass of her and her parachute is 72 kg, then what is the force of air resistance?

a)

She has zero acceleration, so there are no forces.

b)

1400 N in the upward direction.

c)

710 N in the downward direction.

d)

710 N in the upward direction.

3. In the Middle Ages, an archer named William Tell was legendary for his ability to shoot an apple perched on the top of a person's head with his bow and arrow. In order to avoid a hideous accident, how must he orient the initial velocity of his arrow? (Assume that the initial horizontal velocity is sufficient for the arrow to go the necessary distance.)

a)

It must point horizontally.

b)

It must point above the horizon.

c)

It must point below the horizon.

d)

This is the origin of the phrase ``hit-or-miss''.

4. You sit down on your grandmother's cedar chest. How much does the normal force provided by the floor change in this process?

a)

It will increase by an amount given by your weight.

b)

It will decrease by an amount given by your weight.

c)

It will stay the same.

d)

No general conclusion is possible.

{5. A mover pushes two boxes to your right, and in so doing applies a force F. The two boxes are continually in contact. The box on the left has a mass of m, whereas the box on the right has a mass of 3m. What is the magnitude of the contact force which the box on the left exerts on the box on the right? (Neglect friction.)

a)

0

b)

.25 F

c)

.75 F

d)

No general conclusion is possible.

: Problems and Short Answer. Show your work completely to obtain at least partial credit. Read each problem carefully! Each problem is worth 10 points.

6. a) Springtime rains have caused the section of the Missouri River bordering western Kentucky to flood, and you are a rescue worker. You possess a boat which is capable of moving with a speed of 1.5 m/s in still water, and you wish to rescue a girl and her dog who happen to be clinging to a tree rooted in the middle of the river. The river happens to be moving with a current of .60 m/s. The girl and her dog are directly in front of you. How must you orient your boat so that you can move directly across the river to rescue them? Take the shoreline (this is where you start!) to define the x-axis, so that the current flows in the positive x-direction. Report your answer as an angle measured with respect to the positive x-axis. (5 points)

true in

{b) Suppose the current is really a raging torrent, so that the current is 2.0 m/s, and you still possess the boat of part a). Can you still move directly across the river? Why or why not? If not, how must you change your starting position? State your answer briefly, in coherent sentences. A numerical estimate is not required. (5 points)

true in

{7. Consider the following variant of an Atwood machine. The pulley is frictionless, and the ropes are massless in comparison to the masses of the blocks. What is the acceleration of the system? What are  and  ? Use the following values for the masses:  35 kg, and  17 kg. Be sure to define your sign convention for the acceleration.

(Hint: This problem will go faster if you think of the two blocks  and  with the rope joining them as a single object of mass  and then solve for a and  first.)

8. You are a member of an U.N. flight crew sent to deliver emergency medical supplies to an isolated city in Bosnia. The designated drop-off site is in a foggy mountainous region, and your plane's radar isn't working. Thus, you must estimate the correct initial velocity with which to throw the medical supply package, so that it won't miss its mark. The plane's initial velocity is 57 m/s, and the drop-off site is 250 m below. The fog cover lifts slightly, and the pilot tells you that the drop-off site is 310 m away in the horizontal direction. With what initial vertical velocity must you throw the package in order to hit the drop-off site?

Thu Jan 30 14:47:40 EST 1997