your name: _______________________

Physics 121

First Exam -- Solution

October 5, 2006

Instructions: Answer all four questions on these sheets.

Problem I (24 points) is standard multiple-choice, graded only by chosen answer (work needs not be shown).
For Problem II (Lecture demo, 24 points), tell a story in complete sentences. Use descriptive terms so we know what you are talking about. (For example, instead of "it moves forward" say, for example, "the bullet moves to the right".)
For Problems III and IV (26 points each) in principle you may just write down the answer called for (don't forget units if appropriate), but to get any partial credit in case it is wrong, show your method clearly. Nothing taken off for wrong method if answer is right. In any case, leave no doubt which of possibly several numbers or formulas is your final answer -- for example by drawing a box around it.
Use the back of a previous page for scratch work.

The exam lasts 60 minutes. Please hand in all four pages of your paper according to the initial letter of your last name, using the alphabetical trays, except section SEF1 which will have a separate box.

Copy and sign the pledge below:

I pledge on my honor that I have not given or received any unauthorized assistance on this examination.













I. Multiple Choice     Mark your answer in a column on the right,   --   in other words, here   ¯
The correct answers are bold

  1. When we subtract a velocity vector from another velocity vector, the result is:
    a. another velocity.     b. an acceleration.     c. a displacement.     d. a scalar.
  2. A runner runs on a circular track at a constant rate. The greatest change in his velocity
    from his starting velocity occurs:
    a. one-fourth of the way around the track.         b. one-half of the way around the track.
    c. three-fourths of the way around the track.     d. Both a and c are correct.
  3. Jeff throws a ball straight up. For which situation is the vertical velocity zero?
    a. on the way up     b. on the way back down
    c. at the top             d. It is never zero.
  4. Two objects of different mass are released simultaneously from the top of a 20-m tower
    and fall to the ground. If air resistance is negligible, which statement best applies?
    a. The greater mass hits the ground first.     b. Both objects hit the ground together.
    c. The smaller mass hits the ground first.     d. No conclusion possible with the information given.
  5. If we know an object is moving at constant velocity, we may assume:
    a. the net force acting on the object is zero.     b. there are no forces acting on the object.
    c. the object is accelerating.                             d. the object is losing mass.
  6. Which of the following expresses a principle, which was initially stated by Galileo
    and was later incorporated into Newton’s laws of motion?
    a. An object’s acceleration is inversely proportional to its mass.
    b. For every action there is an equal but opposite reaction.
    c. The natural condition for a moving object is to remain in motion.
    d. The natural condition for a moving object is to come to rest.
  7. As I slide a box at constant speed up a frictionless slope, pulling parallel to the slope,
    the tension in the rope will be:
    a. greater than it would be if the box were stationary.     b. greater than the weight of the box.
    c. equal to the weight of the box.                                     d. less than the weight of the box.
  8. The acceleration due to gravity on the Moon’s surface is one-sixth that on Earth.
    An astronaut’s life support backpack weighs 300 lb on Earth. What does it weigh on the Moon?
    a. 1 800 lb         b. 300 lb         c. 135 lb         d. 50 lb
  9. The net force required to accelerate a 20-kg object at 6.0 m/s² on the moon would be
    a. less than on Earth         b. the same as on earth         c. more than on earth
    d. not enough information given
  10. The coefficient of friction between a racecar’s wheels and a level track is 1.0.
    The car's maximum acceleration is
    a. 1 m/s²         b. 9.8 m/s²         c. limited only be the power of the motor
    d. not enouth information given

    Two more multiple-choice question on next page ...

  11. A block is launched up an inclined plane. After going up the plane, it slides back down
    to its starting position. The coefficient of friction between the block and the plane is 0.3.
    The speed of the block when it reaches the starting position on the trip down is:
    a. the same as the launching speed.         b. less than the launching speed.
    c. more than the launching speed.     d. not enough information given.
  12. A box is to be moved across a level surface. A force of magnitude 200 N may be applied at an angle of 30° below the horizontal to push the box or at an angle of 30° above the horizontal to pull the box, either application sufficient to overcome friction and move the box. Which application will cause the box to have the greater acceleration?
    a. the one below the horizontal         b. the one above the horizontal
    c. both give equal acceleration         d. more information is needed
II. Lecture Demo
The picture shows the "funnel cart" that you saw in lecture.
(a) So that's a cart with a spring-loaded funnel running on a track. What was the other important part of the demo (shown faintly if at all in photo)?

There was a ball in the funnel


(b) What happened during the experiment, that is, which parts moved how?

The cart is pushed across the track. At a certain point a bump below the track trips a lever, releasing the spring and ejecting the ball vertically. Because the ball and the cart both move with the same horizontal speed, the ball stays directly above the funnel at all times, and falls back into the funnel.


(c) What physical principle was demonstrated?

The independence of velocity components, in this case horizontal and vertical components.


(d) Was it important that the track was essentially frictionless? What would have happened if there had been a lot of friction slowing the cart down?

Yes, absence of friction was important. Friction on the track would have slowed the horizontal motion of the cart, but not that of the ball while the latter was in the air. The ball would therefore have missed the funnel and fallen somewhere ahead of it





III. Short Answer

A 2000 kg car with weak brakes is on a 30°, 100-m long slope. The brakes apply a retarding force of only 2000 N to the car, not enough to hold it on the slope. So the car starts from rest, moving down the slope. (g = 9.8 m/s²)

(a) How long will it take the car to roll to the bottom of the slope?

Gravity exerts a force mg sin 30° = ½mg = 9800 N down the slope; the brakes apply 2000 N up the slope, so the total force along the slope is 7800 N. This yields an acceleration a = 7800 N/2000 Kg = 3.9 m/s². Since the car starts from rest we have the car's distance along the slope d = ½at²; solve for t:

t = (2d/a)½ = Ö51.3 s = 7.16 s



(b) What will be its velocity at the bottom?

v = at = (3.9 m/s²)×(7.16 s) = 27.9 m/s

IV. Tutorial -- follow this link