What 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 
25oC to 30oC), 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 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 
energy. 

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.