Transcript these slides - Department of Physics | Oregon State

First, some problem-solving hint(s):
Keep in mind that, in any situation where multiple objects are
connected, they’re moving together somehow—they have the same
(magnitudes of) velocity and acceleration.
And conversely: Any object is made of parts that must each obey
Newton’s Laws—and (as noted above) must have the same velocities
and accelerations as all the other parts.
11/2/16
Oregon State University PH 211, Class #17
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Focus: Notice Prep 6-7 problems 3b and 3c….
Don’t forget to do a free-body-diagram of each object in the
system—even in simple-looking problems like this. FBD’s often
don’t take long, but they reveal a lot.
For example, in 3c… even though each cart is moving to the right,
how is it accelerating? What’s happening?
11/2/16
Oregon State University PH 211, Class #17
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A general discussion of strings, tension and pulleys—what are
they, and what do they do? (See also section 7.4 in the textbook.)…
– We will work only with ideal (mass-less) strings this term; they
will connect masses without adding any mass of their own to the
system. So they don’t require any force to accelerate; therefore, the
tension at each point in such a string is the same.
– This is true even when the ideal strings connect masses via one or
more ideal pulleys. An ideal pulley is a mass-less wheel that redirects the tension of a string wrapped along the wheel’s rim. The
wheel is free to turn (without friction), but the string is assumed not
to slip; it travels with the wheel’s rim.
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Oregon State University PH 211, Class #17
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Of course, a free-body diagram will still
reveal what must be true about the forces
acting on each of several objects connected
by ideal strings and ideal pulleys.
Example: One key use of a pulley is to
“multiply” the tension in the string.
Consider this situation, where the block is
hanging at rest. How is FT.A, the tension in
string A, related to the tension in string B?
Do a free-body diagram of the lower pulley
wheel. (It has negligible mass, so we can
ignore the gravitational force on it, but not
the tension forces!)
11/2/16
Oregon State University PH 211, Class #17
String A
FT.A
String B
block
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