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Unformatted text preview: DISCUSSION PROBLEM [4.1]: answer ... Assume wire is horizontal
r
F1 r
F2 and consider a small section
of wire in the middle: r
w DISCUSSION PROBLEM [4.1]:
As the wire is in equilibrium, i.e., no motion, then there is Why does a wire stretched between two posts always sag no net force, i.e., r
r
F1 = F2 in the middle no matter how tightly it is stretched? r
r
... but there are no vertical components of F1 and F2 to
r
compensate w and produce no net vertical force. Therefore, our assumption that the wire is horizontal is
r
rr
incorrect; clearly, to compensate for w , F1 and F2 must
have vertical components. Therefore, they must “slope
upward”, i.e., the wire must sag in the middle, viz:
r
F1 r
w r
F2 1 DISCUSSION PROBLEM [4.2]: ... answer ... DISCUSSION PROBLEM [4.2]: Yikes!
What is the apparent weight of a person in an elevator if N (Upward force from scale) the cable breaks?
w = mg (force due to Earth) From the freebody diagram N − mg = ma v ,
Yikes! so apparent weight N = m( g + a v ) = m( g + ( − g )) = 0 .
Conceptually what’s happening is ... the floor and
weighing scales are falling at the same rate as the person
inside the car so there’s no net force on the weighing
scale.
This is what is popularly known as weightlessness
(except, of course, you still have real weight!). 2 ...
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This note was uploaded on 07/13/2011 for the course PHY 2048 taught by Professor Guzman during the Spring '08 term at FAU.
 Spring '08
 Guzman
 Physics

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