0.23 Phy1210: energy -- internal and external forces  (Page 4/4)

Compress again

Then the ball would compress again and the process of stretching and compressing would continue until the elastic potential energy stored by thecompression of the impact with the floor is expended by transforming it into heat energy, sound energy, etc.

Between impacts with the floor, this process would be occurring in addition to the activity involving gravitational potential energy.

A Slinky toy

A Slinky is a toy that was patented by R.T. James in 1946 and is currently manufactured by James industries.

The original Slinky toy was a coil spring made of spring steel with a diameter of about two inches. I can still remember the first time I saw one"crawling" down a flight of stairs, and I can remember the student that brought it to elementary school to show it off. (Slinky toys are also made of plasticthese days and are made with different diameters.)

If you hold one end of a Slinky toy and allow the other end to fall toward the floor, the type of oscillations described above can easily be observed by asighted person. The bottom end of the spring goes up and down rather slowly, exchanging elastic potential energy for kinetic energy and back againduring each cycle.

Probably discernable by a blind student

If a blind student were to do this with a Slinky toy, particularly with one of the older and heavier versions made of spring steel, the student wouldprobably be able to feel the oscillations as the bottom end of the spring goes up and down.

The blind student could also probably experience the phenomena by gently touching one side of the spring and feeling the individual coils moving up anddown as the spring stretches and compresses.

Conservative versus non-conservative forces

Internal forces are often referred to as conservative forces because they are incapable of changing the total mechanical energy possessed by anobject. In other words, the mechanical energy possessed by an object is conserved when the object is acted upon by internal forces.

External forces, on the other hand, are often referred to as non-conservative forces for exactly the opposite reason. The mechanical energy possessed by an object is not conserved when the object is acted upon byexternal forces. In particular, the mechanical energy possessed by the object will either increase or decrease by the net positive or negative work done onthe object by external forces.

External forces always occur

While discussing physics concepts, we often like to assume conditions such as a friction-free surface, etc. However, there is no such thing as a friction-freesurface. If there were, we could build the ultimate perpetual-motion machine and solve the world's energy problems forever.

A skier on a friction-free surface

If we speak of a skier gliding down a hill on a friction free surface, we can talk about the skier losing potential energy and gaining kinetic energy on theway down. Neglecting air resistance and other external forces, we can say that there is no change in the mechanical energy possessed by the skier on the tripdown the hill. The skier's potential energy is converted to kinetic energy.

Bad for the ski business

But what happens when the skier reaches the bottom of the hill? Some external force must be applied to the skier to cause the skier's velocity to change.(Remember, the velocity of an object can only be changed by the application of a force.)

Without the application of that external force, the skier would continue moving with the same velocity and disappear over the horizon. It would be badfor the ski business if every skier disappeared after only one trip down the ski slope.

A falling object

It has been said that a fall doesn't hurt you. It's the sudden stop (negative acceleration) at the end of the fall that hurts. We can discuss the concept ofonly internal forces acting on an object while it is falling by neglecting air resistance and other external forces.

Eventually, however, that falling object must reach the surface of the earth, the floor, the table top, or some other surface that represents the zeroreference for gravitational potential energy. When that happens, an external force must occur to change the object's velocity and change the mechanical energy possessed by the object.

Resources

I will publish a module containing consolidated links to resources on my Connexions web page and will update and add to the list as additional modulesin this collection are published.

Miscellaneous

This section contains a variety of miscellaneous information.

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