EXAM I, PHYSICS 1403 September 30, 2004 Dr. Charles W. Myles INSTRUCTIONS:

September 30, 2004
Dr. Charles W. Myles
INSTRUCTIONS: Please read ALL of these before doing anything else!!!
1. PLEASE put your name on every sheet of paper you use and write on one side of the paper
only!! PLEASE DO NOT write on the exam sheets, there will not be room!
2. PLEASE show all work, writing the essential steps in the problem solution. Write
appropriate formulas first, then put in numbers. Partial credit will be LIBERAL, provided
that essential work is shown. Organized, logical, easy to follow work will receive more credit
than disorganized work.
3. The setup (PHYSICS) of a problem will count more heavily than the math of working it out.
4. PLEASE write neatly. Before handing in your solutions, PLEASE(!!!): a) number the pages
and put the pages in numerical order, b) put the problem solutions in numerical order, and c)
clearly mark your final answers. If I can’t read or find your answer, you can't expect me to
give it the credit it deserves.
A 8.5’’ x 11’’ sheet with anything on it & a calculator are allowed. Problem 1
(Conceptual Questions) IS REQUIRED! Answer any two (2) of the remaining
problems for a total of three (3) problems required. Problem 1 is worth 34 points.
Problems 2, 3, and 4 are equally weighted & worth 33 points each.
these briefly, in a few complete and grammatically correct English sentences. More
credit will be given for WORDS than for equations!
a. State Newton’s 1st Law and Newton’s 3rd Law.
b. See figure. Suppose that you are riding in a convertible with the top down. The
car is moving to the right (x-direction) at constant velocity v0x . You throw a ball
straight up (from your viewpoint) with an initial velocity v0y while the car continues
to travel forward at v0x. Neglect air resistance. Will the ball land behind the car, in
front of the car, or in the car? WHY? Explain (briefly!) your answer. Use what
you know about projectiles!. Make a sketch of the situation to illustrate your
explanation. Answer the following for a 5 POINT BONUS! During our class
discussion about projectiles, I did an in-class demonstration illustrating the
answer to this question. Briefly describe this demonstration. (If you were
in class the day I did this demonstration, you probably will be able to
answer this. However, if you “cut” class that day, as many of you often do, you
probably won’t be able to answer it!)
c. See figure. A box is sliding at constant velocity (v = constant!) across a flat,
horizontal, frictionless surface. Sketch the free body diagram for this box.
Is there a force in the direction of the box’s motion? Explain your answer
using Newton’s Laws!
NOTE: Answer any two (2) of problems 2, 3, & 4!!!
2. See figure. A person stands a horizontal distance d from the base of a building. He
throws a ball from the ground towards the building. Its initial velocity is v0 = 25 m/s
at an initial angle θ0 = 30º with respect to the horizontal. It hits the building at
a height h above the ground. At the point where it hits, its vertical component
of velocity vy = 0 (that is, at that point, its velocity is entirely in the horizontal
a. Compute the x and y components, vx0 and vy0, of the initial velocity.
b. Compute the time it takes the ball to hit the building.
c. Compute the height h where the ball hits the building.
d. Compute the distance d from the base of the building where the thrower is
e. Compute x and y components, vx and vy, of the ball’s velocity a time t = 0.75 s
after it is thrown. Compute the magnitude and direction of the velocity vector v at
that time.
f. Compute the ball’s height at a time t = 0.75 s after it is thrown.
3. See figure. A mass m = 15 kg is connected to a massless cord and placed on a
horizontal, surface. It is then pulled to the right by a force FP = 45 N that
makes an angle  = 25° with the horizontal. There is no vertical motion. The
coefficient of kinetic friction between the mass and the table is μk = 0.2.
a. Draw the free body diagram for the mass, properly labeling all forces.
b. Compute the horizontal (x) and vertical (y) components of the applied force FP.
c. Write the equations which result from applying Newton’s 2nd Law to the mass in both
the horizontal (x) and the vertical (y) directions. You will receive more credit by writing
these with symbols, without numbers substituted in, than you will by writing them with
numbers substituted in!
d. Compute the normal force between mass and the horizontal surface. Is this force equal
(and oppositely directed) to the weight mg? If so, why? If not why not? Justify your
answer using the appropriate Newton’s 2nd Law equation from part c.
e. Compute the frictional force, Ffr between the box and the surface.
f. Using the appropriate Newton’s 2nd Law equation from part c., compute the acceleration
a of the box. If it starts from rest, how far has it moved after t = 3 s?
4. See figure. Two masses (mI = 15 kg and mII = 25 kg) are connected by a massless
cord over a massless, frictionless pulley as shown in the figure. The masses are
then released, so that mI moves to the right with acceleration a and mII moves
downward with the same acceleration. Assume that friction can be neglected.
a. Draw the free body diagrams for the two masses, properly labeling all
forces (call the tension in the cord FT).
b. The two unknowns are the acceleration, a, of the masses and the tension, FT, in the cord.
By applying Newton’s 2nd Law to the two masses, find the two equations needed to solve
for a and FT.
c. Using the equations from part b, calculate a and FT (in any order).
d. Assuming that it starts from rest, compute the speed of the hanging mass mII
and the distance it has fallen after a time t = 2 s.