EXAM I, PHYSICS 1403
September 30, 2009, 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
3.
4.
shown. Organized, logical, easy to follow work will receive more credit than disorganized work.
The setup (PHYSICS) of a problem will count more heavily than the math of working it out.
PLEASE write neatly. Before handing in your solutions, PLEASE: a) number the pages & put the
pages in numerical order, b) put the problem solutions in numerical order, & 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.
NOTE!!! I HAVE 145 EXAMS TO GRADE!!! PLEASE
HELP ME GRADE THEM EFFICIENTLY BY FOLLOWING
THESE SIMPLE INSTRUCTIONS!!! FAILURE TO FOLLOW
THEM MAY RESULT IN A LOWER GRADE!! THANKS!!
A 8.5’’ x 11’’ sheet with anything on it & a calculator are allowed. Problem 1 (Conceptual)
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, & 4 are equally weighted
& worth 33 points each.
1. REQUIRED CONCEPTUAL QUESTIONS!!! Answer briefly, in complete,
grammatically correct English sentences. I want answers which use mainly ENGLISH
WORDS, NOT symbols or equations! If you insist on using symbols, DEFINE all symbols
you use! ZERO CREDIT will be given for answers with ONLY symbols! For parts a & b:
Newton’s Laws are about forces. Complete statements of each Law MUST mention forces!
If a part contains more than one question, please be sure to answer each one!
a. State Newton’s 1st Law. How many objects at a time does it apply to?
b. State Newton’s 3rd Law. How many objects at a time does it apply to?
c. See figure. A child sits in a wagon, which is moving to the right (x-direction) at
constant velocity v0x. She throws an apple straight up (from her viewpoint) with
an initial velocity v0y while she continues to travel forward at v0x. Neglect air
resistance. Will the apple land behind the wagon, in front of the wagon, or in the
wagon? WHY? Explain (briefly!) your answer. (Use what you know about
projectiles!) Make a SKETCH of the situation to illustrate your explanation.
d. See figure. A hockey puck is sliding (to the right) at
constant velocity across a flat, horizontal, frictionless ice
surface. Which of the sketches in the figure is the correct
free body diagram for this puck? WHY? Explain your
answer using Newton’s Laws! (Hint: Is there a force in the
direction of the puck’s motion?) To answer this correctly,
you need to think like Newton (of more than 300 years
ago) NOT like Aristotle (of more than 3,000 years ago)!
e. Answer the following for 5 BONUS POINtS! During our projectile discussion, I did an
in-class demonstration which tried to illustrate the answer to question c, about the child
and the apple. Briefly describe this demonstration. If you were in class when I did it, you
should be able to answer this. If you “cut” class that day, as many of you often do, you
probably won’t be able to answer it!
NOTE: Answer ANY TWO (2) of problems 2, 3, & 4!!!
2. See figure. At time t = 0, a car is at the origin & is traveling at a velocity of v0 = 47 m/s
along the positive x-axis. It undergoes a constant
v0
t=0
acceleration in the negative x-direction, so it is
v0 = 47 m/s
slowing down. At t = 25 s after it has passed the
t = 25 s
origin, it has slowed to v = 18 m/s.
v = 18 m/s
a.
b.
c.
d.
e.
a
v
Calculate the car’s acceleration.
Calculate the distance the car moved in the 25 s.
Assuming constant acceleration, calculate the car’s velocity at time t = 30 s after it has passed the
origin.
Assuming constant acceleration, calculate the distance past the origin that the car stops.
If the car’s mass is m = 2,000 kg, calculate the total force required to be applied to it to slow it
down & eventually stop it. What Physical Principle (or Law) did you use to do this calculation?
3. See figure. A cannon ball is shot from the ground with an initial velocity v0 = 45 m/s at angle
θ0 = 55° with the horizontal. It lands on top of a building of height h = 46 m above the
ground. Neglect air resistance. To answer this, take x = y = 0
where the cannon ball is shot. Take upward as positive! (Hint:
That the building’s height is 46 m above the ground is
h = 46 m
irrelevant to every question but that in part e!)
a. Calculate the horizontal & vertical components of the initial velocity.
b. Calculate the cannon ball’s maximum height above the ground.
θ0 = 55°
Calculate the time it takes to reach this height.
c. Calculate it’s horizontal (x) distance from the starting point when it has reached it’s maximum height.
d. Calculate the horizontal & vertical components of velocity, vx & vy, after the cannon ball has been
e.
f.
in the air for 5 s.
Use the results of part d to calculate the cannon ball’s velocity (magnitude or length v and
direction θ) after it has been in the air for this same time. (Note: Answers that the angle θ at this
time is the same as the initial angle θ0 will receive ZERO credit! Such answers show a complete
lack of understanding of projectiles & vectors!).
5 POINT BONUS! Calculate the time it takes the cannon ball to land on the top of the building.
When it does so, compute it’s horizontal distance d from its starting point. (Hint: You will need
to use the quadratic equation to answer this!).
4. Note!! Parts a, b, c, & d are about the same box, under different conditions.
The box’s mass is m = 20kg. So, it’s weight mg = 196 N. For a, b, & c, the box
is sitting statically (not moving!) on a flat horizontal table.
Figure a shows the box’s free body diagram when the only forces acting are the
normal force FN upward & it’s weight mg downward. Calculate FN in this case.
b. Figure b shows the free body diagram when, in addition to the normal force FN &
weight mg, an additional downward force FP = 40 N acts on it when someone pushes
on the top of it. Calculate FN in this case.
c. Figure c shows the free body diagram when, in addition to the normal force FN &
weight mg, an additional upward force FP = 40 N acts on it when someone ties a
rope around it & pulls up on it. Calculate FN in this case.
d. Figure d (at the left!) shows the box’s free body diagram when,
similar to the case in Figure c, someone ties a rope around it &
pulls up on it with a force FP = 200 N as shown. FP is large enough
to cause the box to move upward. Calculate the box’s acceleration
Figure d
in this case.
e. In the case of part d, is there a normal force FN acting on the box?
Why or why not? What Physical Principle did you use to answer
parts a, b, c, & d?
θ
Figure a
a.
Ff
Figure b
Figure c