Transcript Class #17 - Department of Physics | Oregon State University

A force F is exerted horizontally on a 20 kg block on a
level surface. The coefficient of static friction between the
surface and the block is 0.25. The coefficient of kinetic
friction between the surface and the block is 0.18. Find
the magnitude of the frictional force acting on the block if
F’s magnitude is: (a) 17.0 N (b) 49.0 N (c) 64.0 N
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1.
(a) 0 N
(b) 0 N
(c) 0 N
2.
(a) 17.0 N
(b) 49.0 N
(c) 35.3 N
3.
(a) 68.0 N
(b) 196 N
(c) 49.0 N
4.
(a) 0 N
(b) 49.0 N
(c) 49.0 N
5.
None of the above.
Oregon State University PH 211, Class #17
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Rolling Friction
When an object is rolling along any surface without
slipping (i.e. we say it has traction), the surfaces don’t slide
past one another, so the traction itself is actually static
friction propelling the object along—helping the rolling.
However, there is another friction force opposing the rolling
motion. This is the rolling friction force, caused by the
molecular “stickiness” of object to surface. Just like kinetic
friction (but usually far weaker in effect—rolling an object
over a surface is easier than sliding it), rolling friction can
be calculated as FR = RFN The coefficient of rolling
friction, r, is a simple, unitless number (often quite a tiny
fraction) that describes how the rolling friction force
compares to the normal force.
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Oregon State University PH 211, Class #17
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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) have
the same velocities and accelerations as all the others.
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Oregon State University PH 211, Class #17
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Focus: 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?
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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? (And see also
section 7.4 in the textbook.)
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Oregon State University PH 211, Class #17
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