# NEWTON'S LAWS

### Bowling Ball Pendulum

A large pendulum is pulled to one side and released from just in front of your face. Upon return it will almost reach your face, so long as you have not moved. This shows conservation of mechanical energy. The sum of the bowling ball's gravitational potential energy and kinetic energy remains constant. As it has no kinetic energy upon release it can get no higher when it returns. When done as the first demonstration it lets you get all of the warnings dealt with at the beginning. I advise putting the back of your head against a wall or upturned table so you know that you have not moved forward.

You can spin around under the headgear while it stays stationary. It demonstrates Newton's first law, and it's cheap. Thanks to Paul Hewitt for this one.

### Paper & Beaker

This is a classic demonstration of inertia, similar to pulling a table cloth from under crockery. It has as much to do with friction as it does inertia. The beaker does not accelerate with the paper as that would need a force much larger than friction can supply. (No one is surprised that the table doesn't move either!)

#### Human Air Puck and the Record Album Mini-version

Human air puck. The puck consists of a four foot diameter plywood disc, the bottom of which is covered by a perforated plastic sheet. You can use either a leaf blower or a shop vac as the air supply. The design is taken from an article that appeared in the November 1989 issue of "The Physics Teacher" page 615. Mini-version. The puck consists of an old record album with an empty sewing thread spool glued to its center. A balloon fits over the spool.

#### Table Cloth and Crockery

Make sure that your cloth does not have a seam along the edge being pulled under the crockery. The crockery will be set in motion, so don't put stuff too near the edge, nor use a table with a very "slick" surface, it's the cloth that needs to be slippery. Your local "thrift" store is a good source of crockery, choose dishes and glasses that have low centers of gravity and smooth bottoms.

#### Sewing Hoop

A more elegant version of the paper and beaker. If the hoop is not warped the pen will only stand up easily when vertically above the bottle's mouth. The sand or lead shot is for stability. It is important that the hoop is struck from the inside so that it forms an ellipse with a horizontal major axis. No friction is involved this way and if there's one thing you can rely on, it's that gravity acts straight down. You can make a "mini" version of this using a section cut from a round, plastic gallon jug and a piece of chalk.

#### Egg & Sheet

This is a graphic demonstration of the effectiveness of air bags and how they relate to Newton's second law. An egg can be thrown very hard at a sheet held by two people. Have the helpers hold the sheet so that it hangs in a 'J' shape. Have some slack in the sheet and don't be afraid to throw the egg hard, but don't squeeze it. A fitted sheet makes catching the egg easier. A water balloon can be substituted for the egg if you're nervous.

#### Bike Wheel Twist

It is very difficult to twist a spinning bicycle wheel, especially if the tire of the wheel is filled with sand so the wheel has a large angular momentum that needs a large torque to quickly change its direction. The wheel needs handles fastened to its axle on either side. Commercial versions are available. Be sure to wear a glove on the hand spinning the wheel and pick someone with fairly long arms and no loose clothing.

#### Bike Wheel and Lazy Susan

Hold a spinning wheel while standing on a turntable. Turn the spinning wheel over, because of conservation of (zero) angular momentum the holder rotates in the opposite sense to the wheel.

#### Skater on Lazy Susan

Hold a sandbag in each hand out at arms length while spinning slowly on a turntable. When you bring your arms into your chest your rate of rotation increases showing conservation of angular momentum. Have your audience just consider the speed of the sandbags. Sandbags are better than barbells as they don't hurt toes as much if dropped.

#### Bike Wheel Gyroscope

A spinning bicycle wheel hung by a chain from the axle on one side, will continue to rotate in the plane it was let go in. A vertical plane works well. It is effectively a gyroscope undergoing precession because of only being supported on one side.