----- The following copyright 1991 by Dirk Terrell
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 Lecture #5  "Mother Nature is a Conservative"

   Next we need to introduce some terms and talk about a few very 
fundamental laws of nature. I find scientific definitions to be much more 
satisfying than those in other areas of human inquiry. As an undergraduate I 
took the mandatory composition course with all of its rules for writing. I 
became very dissatisfied when I would learn the rules only to be told that 
the rule was to be followed except in a situation in which I had used it. 
Naturally the exception hadn't been mentioned beforehand. In physics, 
definitions are more rigorous. Once something is defined, that's the way it 
is from then on. It won't mean something else later on.

   For example, let's take the idea of work. Now that's something we all 
know about! How would you define work? In physics it's easy: work equals a 
force times a distance through which the force is applied. If you push a box 
across the floor, then you do work on the box. If you lift the box up onto a 
shelf, you are doing work. If the box is so heavy that you can't lift it, no 
matter how long you pull on it you will not do any work if it doesn't move. 
You can push on the wall all day and you will do no work. In equation form

                            W=F*d

   Let's say we have two people - one very big and strong, one very small 
and weak. We have 10 boxes of masses 5 kg each and we need to lift them to a 
shelf 1 meter above the ground. We set the two of them in motion and the 
strong person finishes rather quickly. The weak one struggles a bit but 
finally finishes a while later. Which one does more work? Most people are 
tempted to say that the weak person does more work, but in the scientific 
spirit of work, they both do the same amount of work. They have applied the 
same force through the same distance. But if you own a trucking company, who 
do you hire? The strong person of course, because he can do a certain amount 
of work in a shorter period of time. In physics we refer to work done per 
unit of time as power and in the metric system the unit of power is the 
watt. The unit of work is the joule. Therefore since power=work/time, a watt 
is one joule of work per second. Your 100 watt light bulb consumes 100 
joules every second. If you lift a 1 kg mass through a distance of one meter 
you do about 10 joules of work. That means you would have to lift 10 kg a 
distance of one meter every second to equal the power output of the light 
bulb. Think about that the next time you leave the lights on when you leave 
a room! The equation for power is 

                            P=W/t

   Energy is something we hear a lot about, but what is the scientific 
definition of energy. Energy is defined as the ability to do work, and thus 
energy has the same units as work- the joule. When you do work, you expend 
energy. One very important law in physics is that energy cannot be created 
or destroyed. It is sometimes referred to as the law of conservation of 
energy. Every single experiment ever done to test the law has verified it. 
Perpetual motion machines are attempts to violate this law. None have ever 
succeeded.

   Momentum is another word we use in everyday language. What do we mean by 
momentum? We refer to it when observing collisions between objects. What 
does momentum depend on? It seems to depend on the masses of the colliding 
objects. Most of us wouldn't worry about being hit by a mosquito flying 
along at 10 mph, but I doubt we would be so willing to get in front of a 
tractor-trailor travelling 10 mph. It also seems to depend how fast the 
objects are travelling. If someone tossed you a baseball at 10 mph, you 
would probably be able to catch it bare-handed. I wouldn't want to try to do 
that with a 95 mph Dwight Gooden fastball!. In physics, momentum is mass 
times velocity, or

                           p=m*v

(Yes, p stands for momentum. I'm not sure how it was decided to use p.)

   Momentum is another conserved quantity in nature. Conserved quantities 
are very useful because they enable you to calculate many useful quantities. 
The total momentum in a system before collisions occur equals the total 
momentum after the collisions. We will see examples of momentum conservation 
all over the place. Ice skaters make use of the conservation of angular 
momentum when they pull in their arms to speed up when spinning and put out 
their arms to slow down their rate of spin. You can test it out by sitting 
on an stool that can be spun, with your arms and legs held out. Now have 
someone spin you around (slowly!). Now pull in your arms. What happens? If 
you're still on the stool, pull in your legs. What happens? Now extend your 
arms and legs again. What happens?

 Dirk