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Lecture 10

PHYA10H3 Lecture Notes - Lecture 10: Friction, Gyroscope, Opata Language

Physics and Astrophysics
Course Code
Brian Wilson

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Note-Taker: Facilitor:
PHYA10 Practical Week 10
1. (a) Below is a picture of a yo-yo. If you put it on a table as shown and pulled the
string, what direction would it travel? Explain why.
(b) If instead you had the rope coming from under the bottom rather than over the
top, would it still travel in the same direction? Explain why or why not.
(c) Once you have shown your predictions to parts A and B to your TA, your TA
will let you borrow a yo-yo. Test your predictions and describe the results. If
necessary, explain why your predictions were incorrect.
(d) Use these results to describe what should happen if you pull the string at various
angles. There is a particularly interesting angle. Why is it interesting? Explain
what happens when you pull the string at that angle, and explain why.
2. Your TA has a gyroscope and a string. You can use the string to set they gyroscope
spinning very quickly. When you get the hang of that, try the following two things:
(a) Hold one end of the frame with just two fingers and a thumb while the gyroscope
is spinning fast. Hold it upright, then quickly try to flip it 180. Describe what
you feel.
(b) When the gyroscope is spinning fast it is very stable. It can be held up against
gravity in some strange ways which are impossible when it is not spinning. Exper-
iment with this and describe a few of the bizarre configurations which are stable.
There should be a common occurrence with all of these situations which is related
with your experience in part (a). What is it?
(c) Explain how the right hand rule and ~τ =~r ×~
Fand ~τ =Id~ω
dt is the physics behind
your answers to the first two parts (if you have not already done so). Specifically,
how do the directions of ~
Fand d~ω
dt compare with each other?
3. A cube rests on a ramp. The angle of the ramp is going to slowly increase until the
ramp becomes vertical. What is the minimum coefficient of static friction required to
ensure that the cube will start rolling before it starts slipping?
4. A geosynchronous orbit is when a satellite orbits the Earth with a period of exactly 1
day. As a result, that satellite appears (from the rotating Earth’s non-inertial reference
frame) to stay in one location in the sky. This is convenient for satellite television and
radio stations. Find: how far from the Earth the satellite must be; how fast it is
traveling; and what the escape speed is (i.e. how much faster it would have to move
in order to leave the Earth forever). You can ignore the Sun and the Moon for this
question. Finally, if the satellite was a little farther away, would it appear to move
from East to West, or from West to East, to an observer at rest on the Earth? What
if it was a little closer?
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