Py 120 Test 3 Spring 1998

Name: ______________________________________

Pledged:_____________________________________

Part A: Interactive Type Problem. (20 pts)

1) A traveling pressure wave is shown on the monitors in the front of the room. An
observer is stationed at a distance D from the wave source and measures the pressure as a
function of time. Her measurement of the pressure variation as a function of time is shown
above for a two milliseconds time duration. Assume that the speed of sound is 330 m/s.
Answer the following:

a) What is the average frequency of the sound heard?

b) What beat frequency is heard?

c) Write an equation for the pressure variation as a function of time at D.

d) Assume that intensity is momentarily zero at D. Are there other points

where the intensity is a maximum at that same instant? If so, where?

Part B: Concept problems. (20 pts each.) These are short but you must still GIVE YOUR REASONS.

- A mass is attached to a spring and caused to oscillate. Is there an instance in time when both velocity and acceleration are zero? If so, when does this occur. Is there an instant when velocity is zero and acceleration is negative? If so, when does this occur? Sketch the position and velocity as functions of time for two cycles.

- Two solid disks initially at rest roll down an incline plane from the same initial height. Both disks have the same mass but disk B has twice the radius of disk A. Derive an expression for the ratio the velocities of the disks at the bottom of the incline. Ignore friction.
- A 300 gram mass vibrates 4 times per second with an amplitude of 10 cm. Assume that it
has its maximum velocity at time t=0.

a) Write Newton’s second law for the mass.

b) Write an equation for the position of the mass as a function of time.

c) Determine the velocity of the mass when it passes through the equilibrium point.

d) Find the energy of the system at t=0.

A 500 gram rock is suspended at the end of a 1 meter stick as shown above. The stick is
marked in 25 cm increments. Where is the center of mass of the system? How much does the
meter stick weigh?

Part C: Textbook problems (40 pts total.)