NSC 101 Laboratory for 2/17/98

EXAM I

The first exam will cover Chapters 1,2 3, and 5. Chapter 4 will be covered on Exam II. Below are a few sample questions for the first exam. You will be provided with equations, so don't bother memorizing them. LEARN HOW TO USE the equations.

Look over the homework exercises and problems, the problems we worked in class and the quiz problems. Almost any of the exercises or problems not assigned also make for good practice. Start studying now. Don't wait for Monday night! Good Luck.

  1. You toss a rock directly upwards and it returns to your hand. Ignoring air resistance, circle all the correct statements below.
    1. The acceleration is zero at the top of the flight.
    2. The velocity is positive throughout the flight.
    3. The time rising upward equals the time falling downward.
    4. The acceleration reverses direction at the top.
    5. The rock returns to your hand with the same speed it left your hand.

  2. Three different forces are acting on an object. Can you say whether or not the object is accelerating? Explain.

  3. A small car collides head-on with a large truck. Which, if either, experiences the greater force. Which, if either, experiences the greater acceleration?

  4. A tire at the end of a rope swings back and forth. Explain what is happening in terms of energy.

  5. A 2 kg block of balsa wood floats with exactly half of its volume submerged. What weight, placed on top would cause it to just be completely submerged? Explain your reasoning!

  6. Explain how the shape of an airplane wing creates the lift necessary to support the weight of the plane.

  7. A bicycle courier rides her bike 200 m east, then 300 m north in a total time of 1 minute.
    1. What is her average speed?
    2. What is her average velocity?

  8. An 80 kg skydiver at one point has an acceleration of 5 m/s2 downward.
    1. Draw all the forces acting on the diver in a free-body diagram.
    2. What is the value of the air resistance acting on the diver at the instant in a)?
    3. What will be the value of the acceleration when he reaches terminal velocity?
    4. What will the air resistance be then?

  9. Two 1000 kg cars are traveling at 10 m/s towards each other.
    1. If the cars collide and stick together, what is their velocity just after the collision?
    2. Was kinetic energy conserved in the collision? If not, what happened to the KE?

ANSWERS: Spend at least a half hour working on the answers before looking!

  1. Only c and d are correct.

  2. You can't say anything about the net force without knowing the magnitude and direction of the individual forces. They may or may not add to zero. So the acceleration may or may not be zero.

  3. They both experience the SAME FORCE!...Newton's Third Law. But the less massive car will experience the greater acceleration. (F = ma).

  4. The tire's energy is all potential at the top of the swing, with no kinetic energy. As the tire swings downward, the PE is converted to KE. At the bottom of the swing, the energy is all KE.

  5. Exactly 2 kg will cause it to be completely submerged. If it is floating, then the mass of the water displaced must be equal to its own mass...2 kg. Since one half of its volume is submerged, then the mass of the water displaced when completely submerged must be another 2 kg.

  6. The wing is shaped such that air must travel a greater distance over the top, causing the air to travel faster over the top than the bottom. Higher velocity over the top of the wing means less pressure than on the bottom. The greater pressure below the wing results in the lift.

  7. average speed = (200 m + 300 m)/ 60 sec = 8.3 m/s (500m/min is also acceptable)
    average velocity = (square root of 200 squared + 300 squared)/60 sec = 6.0 m/s NE

  8. The force diagram should show her weight (W) acting downward and air resistance (Fair) acting upward.
    F net = ma. Take down to be negative. Then, Fnet = Fair-weight = ma. So, Fnet = Fair-(80 kg)(10m/s) = (80kg)(-5m/s2). Solve for Fair = 400 N (half her weight!) If you took down to be positive (which is ok!), then all the signs would be reversed.
    At terminal velocity, her velocity is constant, so accleration is zero. So, F air must be just equal to her weight = 800N.

  9. Conservation of momentum ... p initial = p final ---> (1000 kg)(10 m/s) + (1000 kg)(-10 m/s) = (2000kg)(V final). So, V final = 0!
    KE was not conserved. The KE went into deformation, heat, sound, etc.