Lecture Demos: Mechanics

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Dropped Slinky

Dropped Slinky

A Slinky is held by one end so that the other end is just off of the floor (this will probably require standing on the bench or similar object). The demonstrator then releases the Slinky to show the contraction.

PIRA Code(s): 
1G20.45
Set up time: 
2 minutes
Physics and Science Concepts: 
Newton’s Second Law
Operation: 
Hold a slinky in your hand high enough so that when it falls it will not touch the floor. Still holding the top of the slinky, let the rest drop down towards the ground.
Safety: 
Ends of slinky may be sharp or become tangled

Monkey and Hunter

Monkey and Hunter

This experiment illustrates the story of a hunter who tries to shoot a monkey. When he fires the gun, the monkey is scared, lets go of the branch, and begins to fall. The projectile is fired directly at the initial location of the monkey. Because the monkey begins to fall at the same time as the projectile is fired, and due to the independence of the horizontal and vertical components of motion, the projectile falls “with” the monkey and strikes the monkey. Both the monkey and the bullet accelerate downwards at the same rate due to gravity.

PIRA Code(s): 
1D60.30
Set up time: 
10 minutes
Physics and Science Concepts: 
Projectile Motion, Gravity, and Mechanics
Operation: 
Set up two ring stands about 8 feet apart. Put the blow gun in the clamp on one stand with the power cords to the inside of the set up. Attach the pole with the magnet to the other ring stand. Place the ball into the blow gun. Switch the power supply on which connects the circuit and then place the monkey (band aid box) on to the magnet, upside down. Make sure the blow gun is aimed directly at the broad side of the monkey. Now, blow into the blow gun to release the ball and break the circuit, thus causing the monkey to fall. The ball should hit the monkey as it falls.
Safety: 
Do not turn power supply on before inserting the ball into the blow gun, due to possible electrocution. Not all pieces (ring stand, 5 V ½ Amp DC power supply, and clamps) are in the kit, these items need to be acquired.

Simultaneous Fall

Simultaneous Fall

An apparatus is used to launch a ball and drop a ball simultaneously to show that they hit the table at the same time.

PIRA Code(s): 
1D60.20
Set up time: 
5 minutes
Physics and Science Concepts: 
Projectile Motion
Operation: 
Mount apparatus to ring stand using clamp, adjust until apparatus is horizontal, cock the apparatus spring, and place the masses (one at the end of the apparatus and one on the bar that extends from the apparatus), release, and one ball will fall from rest (hung on the rod) while the other ball is struck by the rod and has an initial horizontal velocity. The balls should hit the table at the same time, which can be heard clearly by students. Repeat by cocking spring to different compression. Repeat by clamping apparatus at a different height to show that height has no effect on the simultaneity of the fall.
Safety: 
Once spring is compressed, do not put hands in front of the end of the rod.

Howitzer

Howitzer

Ball is fired vertically from a moving cart and falls back into cup.

PIRA Code(s): 
1D60.10
Set up time: 
1 minute
Physics and Science Concepts: 
Mechanics, Motion in Two Dimensions, Projectile Motion
Operation: 
Press down on the inside of the cup. Turn the cart over, and stick the pin in the hole that protrudes from the bottom of the cart. Place the ball inside the cup. Then, push the car at a constant velocity as you gently pull the string. The ball should pop up and land in the cup again.
Safety: 
None

Brachistochrone Track

Brachistochrone Track

Three tracks are straight, parabolic and cycloid. All three paths have the same initial and final states. When the balls race down the curves the cycloid path arrives first. The parabola is a very close second. The straight line is a distant third although many students think the straight path should have the fastest time.

PIRA Code(s): 
1D15.45
Set up time: 
1 minute
Physics and Science Concepts: 
Motion in Two Dimensions
Operation: 
Place balls of equal mass and size at the top of the tracks, one at the cycloid, one at the parabola, and one at the straight track. Let the balls roll down, the cycloid ball will arrive first. Then, place two balls on the cycloid track, one at the top and one a little ways down. The two balls will reach the end at the same time.
Safety: 
None

The Penny and the Feather

A tube with pennies and feather inside has all the air taken out of it. When this happens, the penny and feather will fall at the same rate.

PIRA Code(s): 
1C20.10
Set up time: 
1 minute
Physics and Science Concepts: 
Uniform Acceleration, Mechanics, Gravity
Operation: 
Tip the tube with penny and feather upside down. The penny will fall faster than the feather. Then, use the pump to evacuate all of the air. Like before, turn the tube upside down and the two objects will fall at the same rate.
Safety: 
Do not crack tube. Must check out demonstration 3B30.30 to obtain and use pump.

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