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Per 8 Activity Solutions: Work, Efficiency, and Complex Machines
8.1 How Much Work is Done by Machines?
How Efficient Are They?
a)
Block and Tackle
Your instructor will use the block and tackle to lift blocks.
1) How much weight did the block and tackle lift?
_______________
2)
How high off the floor were the cement blocks raised?
_______________
3)
Find the work done on the blocks to raise them.
This is the
work out
.
Work
out
=
F
out
x
D
out
___________
4)
How much force was needed to raise the blocks?
_______________
5)
Through what distance was the rope pulled to lift them?
________________
6)
Find the work done when you pull the block and tackle rope.
This is the
work in
.
Work
in
=
F
in
x
D
in
_____________
7)
Calculate the efficiency of the block and tackle.
in
out
Work
Work
Efficiency
=
b)
Winch
Try a “tug of war” game with the winch in the front of the classroom.
1)
Suppose you move the handle of the winch in a circle a distance of 112 meters
(distance in).
When you do, the winch rope moves 2 meters (distance out).
If you exert a force of 280 newtons on the winch handle (force in), calculate how
much work in must you do to move the winch handle a distance of 112 m.
Work
in
=
F
in
x
D
in
=
280 N
x
112 m
=
31,360 joules
__
3.14 x 10
4
J
__
2)
If the force out on the rope is 8,400 newtons, how much work out does the
winch do as it pulls the rope a distance of 2 m?
Work
out
=
F
out
x
D
out
=
8,400 N
x
2 m
=
16,800 joules
__
1.68 x 10
4
J
___
3)
Calculate the efficiency of the winch.
Efficiency
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 Spring '10
 Staff
 Energy, Force, Potential Energy, Work, Simple machine, mechanical advantage

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