Return to Physics IndexWhat Is Work?

Economou, Alex Chicago Vocational H. S.

Objective:To show work (w) equals force (f) times distance (d) and the connection between work and potential energy, kinetic energy, and heat.Equipment: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 25^{o}C to 30^{o}C), two beakers, ice, boiling water, a thermoelectric

converter (PASCO SCIENTIFIC, 1876 SABRE STREET, HAYWARD CALIFORNIA,

94545. 1-800-772-8700)Procedure: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 atconstant speedthe 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

energy.

We know that V^{2}_{f}-V_{o}^{2}= 2*A*S = 2*g*h. If V_{o}=0, then V^{2}_{f}= 2*g*h.

Multiply both sides by m. mV^{2}= 2*m*g*h or mV^{2}/2 = m*g*h. This states

that mV^{2}/2 at end is the same as the m*g*h at the start. This

mathematical quantity, m*V^{2}/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 V^{2}/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.