What Is Work?
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Economou, Alex Chicago Vocational H. S.
To show work (w) equals force (f) times distance (d) and the connection
between work and potential energy, kinetic energy, and heat.
Single and double pulleys to produce mechanical advantages of 2 to 5,
stands and supports for such, string masses from 0.1 to 2 kg, N force
scales to 5N, wooden inclined plane about 2 m long, a wood disk or
block that can support masses up to 10 kg, a liquid crystal strip about
20 cm long 5 cm wide (the strip should have a temperature change at
25oC to 30oC), two beakers, ice, boiling water, a thermoelectric
converter (PASCO SCIENTIFIC, 1876 SABRE STREET, HAYWARD CALIFORNIA,
Set up a number of pulley assemblies depending on the number of class
experimental groups each with a different mechanical advantage and
mass. Determine the force needed to lift at constant speed the provided
weight, the distance the weight is lifted, and the distance the force
at the other end of the string moves as the weight is lifted. A data
table can be constructed thus:
mass of weight of distance force used distance column column
object object object is to lift the force 2*3 4*5
lifted lifted is lifted object travels
kg N m N m
Column 6 should be equal to column 7. But column 6 is mgh and column 7
is f*d. Point out that lifting the weight is tiring, because you have
done work. How does that work appear on the data table? What is
constantly the same in each row? (f*d = mgh) We therefore define
work done as f*d. (If f is in Newtons and d is in meters, work has
units of Joules.) Mgh is also the work done, but that raised weight
has the work somehow in it. Demonstrate this by dropping a weight on
piece of chalk. Point out that the weight did work on the chalk, and
therefore has the ability to do work when raised. We call that ability
because of its position potential energy. Point out that the weight,
just before it strikes the chalk has no potential energy. Since we
have shown that the work we did was converted to potential energy, then
it follows the potential energy was converted to something. Since the
weight was moving, the new energy is energy of motion, called kinetic
We know that V2f-Vo2 = 2*A*S = 2*g*h. If Vo =0, then V2f = 2*g*h.
Multiply both sides by m. mV2 = 2*m*g*h or mV2/2 = m*g*h. This states
that mV2/2 at end is the same as the m*g*h at the start. This
mathematical quantity, m*V2/2 is called kinetic energy, KE.
Next set up an inclined plane so that weights piled on the wooden disc
or block will not accelerate as they slide. Tape a liquid crystal
strip to the plane's lower third. Demonstrate that the strip changes
color with temperature by touching it. Slide the weights and support
down the plane. The strip changes color showing heat was produced.
Point out that the KE of the weights as it slid down the plane did not
change (no change in speed) but the potential energy did. Why? If we
believe that the energy at first equals the energy finally then we must
add heat to make a balanced statement;
m*g*h (at first) = m V2/2 (at end) + heat.
Heat must be a form of energy.
To show that heat can be used to make KE and work use the
thermoelectric converter, one plate in hot water the other in ice
water, in different beakers.