# Mechanics

#### 4) It's particularly effective when they see a bowling ball and a marble "tie" - a great demo of "variable independence"...

##### "Levitating" Spheres
1) Get a bowling ball and 3 - 6 students
2) Have the students stand in a circle at equally-spaced positions around the bowling ball and try to lift it upwards together over their heads, each student using only his/her index finger in the process.
3) They will find this very difficult at first, but then the ball will "magically" feel as though it's "levitating".
4) Have the students explain this using the ideas of vectors.

## Yardstick Center of Mass

Place two fingers anywhere underneath, and bring them together. They will always meet under the center of mass.
contributed by N. Milburn

You can turn this into a card trick. Place several cards in card holders on a meter stick. "Force" a match of the card that is at the center of mass and then have a student reveal it by using "the magic of physics." Then challenge students to determine a way that the trick could have worked. (Check a magic book out of your library or e-mail me at jbuchman@fmschools.org if you need help with forcing a card.)
contributed by J. Buchman

## Dueling Dart Guns

Get two dart-guns, each with a dart. Affix a lump of clay to one dart. Fire them simultaneously pointed down at the floor from as high a location as you are able. But first ask them: which one will hit the floor first?
contributed by D. Baird via B. Taylor

Again, from a high location: from one hand let a slinky hang, the other hand any solid object. Drop them both at the same time - but first ask: which one will hit first? (Falling faster than g). Try it again, and ask: how far down the slinky should I position the solid object so that they hit at the same time? Half way down?
contributed by P. Doherty via B. Taylor

## Table Cloth and Inertia

Party Tricks:
How To Pull A Tablecloth From Under A Dinner Service

Insert description...

## Artificial Gravity Problem

This problem is a real thinker - great once kids are getting the idea of artificial gravity. The full problem with illustrations and the People's Physics Book can be found here. Contributed by Bill Taylor.

A space station was established far from the gravitational field of Earth. Extended stays in zero gravity are not healthy for human beings. Thus, for the comfort of the astronauts, the station is rotated so that the astronauts feel there is an internal gravity. The rotation speed is such that the apparent acceleration of gravity is 9.8 m/s2. The direction of rotation is counter-clockwise.
a. If the radius of the station is 80 m, what is its rotational speed, v?
b. Draw vectors representing the astronaut’s velocity and acceleration.
c. Draw a free body diagram for the astronaut.
d. Is the astronaut exerting a force on the space station? If so, calculate its magnitude. Her mass m = 65 kg.
e. The astronaut drops a ball, which appears to accelerate to the ‘floor’, (see picture) at 9.8 m/s2.
i. Draw the velocity and acceleration vectors for the ball while it is in the air.
ii. What force(s) are acting on the ball while it is in the air?
iii. Draw the acceleration and velocity vectors after the ball hits the floor and comes to rest.
iv. What force(s) act on the ball after it hits the ground?