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Work and the Work Energy Theorem

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Phu Trinh

on 18 December 2013

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Transcript of Work and the Work Energy Theorem

Work Theorem
Sample Questions
Work and the Work Energy Theorem
Work- requires that a force be applied and that this force be applied over some displacement
Useful Equations for Solving
Sample Question 1
Sample Question 2
A 5.6 N weight is lifted 2.2 meters. How much work is done?
Sample Question 3
A weightlifter lifts a 2000 N weight to a height of 2 m in 1 second. Another weightlifter lifts a
2000 N weight to a height of 2 m in 2 seconds.
(a) Which weightlifter did more work?
(b) Which weightlifter required more power?

An 80 kg object is to be pulled to the top of a 6.0 m ramp by a rope, the other end of
which is pulled up by a 2400 W electric winch. The ramp forms a 30° angle with the
horizontal. The coefficient of kinetic friction between the ramp and the object is 0.6 and
the coefficient of static friction is 0.8. The object moves up the ramp at a constant
Free Response Question Solution
To Calculate Work- multiply the force applied by the displacement
Units used- Joules
Work is scalar
There are two ways of multiplying vectors:
Cross product- Gives vectors as an answer
Dot Product- Gives a scalar as an answer
Work Energy Theorem-
If work is done on an object, some of the work results in the displacement or position change of the object
1. Suppose a car with a miracle engine is able to convert into work 100% of the energy released
when gasoline burns (40 million joules per liter). If the air drag and overall frictional forces on
the car traveling at highway speed is 500 N, what is the upper limit in distance per liter of
gasoline the car could cover at highway speed?

W = Fd  d = W / F = 40,000,000 J / 500 N = 80,000 m = 80 km
W = Fd = (5.6 N)(2.2 m) = 12.32 J

First lifter: W = Fd = (2000 N)(2 m) = 4000 J
Second lifter: same
First lifter required more power:
P = W / t = 4000 J / 1 s = 4000 watts
Second lifter: P = 4000 J / 2 s = 2000 watts

Free Response Question
A. On the diagram of the object below, draw vectors
showing all the forces on the object. Label each one.
B. Determine the maximum constant speed at which the winch can pull the object up the
C. Determine the total amount of work that the winch must do in pulling the object up the
Full transcript