Physics 116a02: Extra Hints for Assignment 7
Department of Physics and Astronomy, Vanderbilt University
Things to Think About for Assignment 7
- All Work and No Play
- No extra hints needed. Parts H through K refer to the Intro 2 figure, and
you should only give answers to two significant digits.
- Hooke's Law
- For part A: My first answer included a comma to denote the thousands
position. However, the program is not smart enough to recognize the comma
for this purpose and marks this as a wrong answer.
On the other hand, in much of the rest of the world,
they use a "decimal point" where we use a comma in the U.S., and similarly
they use a comma to mark the decimal places instead of the "decimal point"
which we use. This program does use the decimal point to mark the decimal
places.
- For part B: The 25 people includes the driver, and a three signifcant
figure answer is required.
- For parts C and D: At least you will learn a little more about
light truck suspension systems.
- Understanding Work and Kinetic Energy
- No extra hints needed. Don't be surprised at the pattern of your last few answers.
- Work from a Constant Force
- For parts A and B: You think about whether you expect the work to be
positive or negative in each case.
- Work-Energy Theorem Reviewed
- For part B: Assume that the constant force is always in the same direction
as the displacement.
- For part D: The force vector in the integrand does not have to be always
in the same direction as the displacement.
- For parts E and F: The force is in the direction of the displacment D.
- Pulling a Block on an Incline with Friction
- When Push Comes to Shove
- For parts A and B: Warning that the distances are being quoted in
centimeters, and the answer is in units of Joules.
- Exercise 6.2
- For parts C and D: You might not have thought at first that there was work being
done on the tow truck. How do you interpret these results, assuming that they
are not zero?
- For part E: Would your answer change for the case where the cable was at
35 degrees above the horizontal?
- Exercise 6.11: Meteor Crater
- Exercise 6.20: Moving Electron
- For part A: You are being asked to find the net work done on the
electron. If you give your answer in fractional terms (which is the most
precise actually), then the program will complain about a non-existent
rounding problem. No points will be deducted.
- Exercise 6.27: Stopping Distance
- No extra hints needed. Normally, a tire is momentarily at rest when it
is in contact with the road's surface, even when braking We will see that in the chapter on
rotations. So normally, it is the larger static coefficient which is helping
to brake the car. However, if you try to brake too hard, then the tires
begin to slide or skid (locked wheels), and your stopping distance increases by the
ratio of the static to the kinetic friction coefficients. Your best action, if possible, is
to stop braking and let the wheels regain their rolling motion. Then you can
apply the brakes more gently.
- Exercise 6.37: Work with a Varying Force
- Exercise 6.44: Electric Consumption
- For part B: The answer is in units of kW/person.
- For part C: For comparison, the total area of the United States is 9,826,630 square km.
- Exercise 6.67: Varying Coefficient of Friction
- No extra hints needed. This is not a simple plug-in answer, and you really
do not need to know the mass.
- Exercise 6.42
- For part A: There are two ways of thinking about this problem. The first
way, using the Chapter 6 Kinetic Energy theorem, is to compute the sum of the work done
by the spring and the work done by gravity. Since the brick starts out with
0 speed and ends up with zero speed, then the sum of these two works must be
zero. The second method, is to use the conservation of mechanical energy
law described in Chapter 7. In that method, the potential energy of the
compressed spring is converted into gravitational potential energy of the
brick.
- For part B: You can write a sentence or two, but there will be no grade
by the program. I'll look at these answers, and discuss the correct answer in class.
- Exercise 6.14
- For parts D through F you can think of air resistance as being a
frictional force which opposes the downward fall of the watermelon.
- Exercise 6.16
- No extra hints needed. Did you really need to know that the rock's weight
was 20 N?
- Exercise 6.29: Stretching and Compressing a Spring.
- Exercise 6.24: Hitting a baseball
- For part D: You will write one or two sentences, but not receive a grade
from program. I will look at your answers, and discuss the answer in class.
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This page was last updated on February 13, 2008