0.21 Phy1190: energy -- kinetic and mechanical energy  (Page 5/8)

A moving hammer

For example, a moving hammer can cause a nail to be displaced and driven into a piece of wood. In the case of a hammer, simply laying the hammer on the head of the nail,which means applying the hammer's potential energy to the head of the nail, probably won't displace the nail. However, giving the hammer a good swing,thereby imparting kinetic energy into the hammer, gives the hammer the capability to displace the nail.

Potential energy being dissipated

If you swing a hammer repeatedly for the purpose of driving nails, after awhile you willprobably begin to feel tired. This is because each time you swing the hammer and impart mechanical energy into the hammer, that energy is being expended by yourbody. After awhile, your available potential energy in the form of chemical energy will have been depleted (or at least reduced) and you will feel tired.

The suction- cup dart gun

When I was a child, a commonly available toy was a gun that was designed to shoot projectiles at a target. The projectiles (darts) were sticks with rubber suction cups on one end. (Note that darts also come in another form that has a sharp but sturdy pin on one end and aconstruction that is very aerodynamic. Those darts are meant to be thrown at a target instead of being shot from a gun.)

The suction-cup dart

For the suction-cup dart, the target was very smooth. If the dart struck the target straight on with very little angle, the suction cup would adhere to thetarget by squeezing air out from the suction cup and creating a weak vacuum.

Cocking the gun

To prepare the gun for shooting the dart, the stick would be inserted into the barrel of the gun and pushed against a spring contained inside the gun until some sort of catchingmechanism would click into place. That mechanism would hold the spring in a compressed position until the child pulled the trigger.

With the spring compressed, the gun possessed mechanical energy due to the elastic potential energy of the spring.

Shooting the gun

When the trigger was pulled, the catch would release, the spring would expand, and the dart would shoot out of the gun at relatively high speed.

Be careful what you aim at

I still recall getting in trouble with my mother for lying on my bed and shooting dartsat the ceiling, which made little round circles on the ceiling that she had toclean off. (I forgot to mention that you normally lick the suction cup before loading it into the gun so that it will make a good seal when it strikes thetarget.)

An exchange of mechanical energy

When the dart is loaded into the gun and the spring is compressed, the gun possesses mechanical energy. When the trigger is pulled, that mechanical energy does work on the dartcausing the dart to be displaced very rapidly shooting out of the barrel of the gun. The elastic potential energy in the spring is exchanged for kinetic energy in thedart.

Another energy exchange

When the dart strikes the target, the kinetic energy in the dart is exchanged for elastic potential energy in the rubber suction cup on the front of the dart. When thestriking angle is nearly perpendicular to the target, the air is forced out of the suction-cup causing the dart to adhere to the smooth surface.