## Rolling Cone

Rolling means moving along a surface by revolving, or turning over and over, and without sliding. When a cylinder rolls, it tends to roll in a straight line. But if one end is wider than the other, it will go around in a circle. I used to call this technique the “Differential Compass”, but now I just call it the “Rolling Cone”

There are many things that might be lying around your house that can act as a rolling cone. See what you can find.

The wider side travels further than the narrower side with each rotation of the cone. If both sides were the same size, it wouldn’t be a cone, and it would roll in a straight path. Try this with other objects, and observe how the path changes, as you change the cone.

All complicated machines are made from simpler parts. If you haven’t mastered this technique, or don’t yet understand it, break it down into smaller parts till you “get” each one, and then build up from there. Find or make a cone shape that will roll on a curve. Input force on one end, and the cone will output force at the end of its curved path. Place the cone on a surface slightly higher than the table, like a domino, which stores a bit of potential energy, and adds to the force of the rolling cone when that potential energy is converted to kinetic (movement) energy.

## Popsicle Stick Catapult

A ping-pong ball sits on a tiny rubber band (a rainbow loom, or hair elastic) at the end of a popsicle stick. That provides friction to keep the ball in place. A small rod, spool, or domino acts a fulcrum. The catapult is a simple lever – a rigid object (popsicle stick) rotating around a fixed point, the fulcrum. Like a seesaw, but notice the fulcrum is not in the middle. The short end gets the force, and the long end sends the ball flying!

You can use a plastic spoon, or pretty much anything with a flat or rounded surface. The greater the incline (tilt), the higher the ball can go. Too much incline though, and the ball will roll off. As in the story of Goldilocks and the three bears, engineering (and life) is a dance between too much and too little, we’re always looking for the “just right”.

The potential energy is in the position of the block that is set to fall on the short end of the catapult. The higher and heavier the block, the greater the force. Newton’s second law says that for every action there’s an equal and opposite reaction. With a lever, as one side goes down, the other side must go up (conservation of momentum) as it rotates around a fulcrum. And we exchange force for distance, so the reaction (the flying ball has lots of distance) is equal to the action (the falling block has lots of force).

All complicated machines are made from simpler parts. If you haven’t mastered this technique, or don’t yet understand it, break it down into smaller parts till you “get” each one, and then build up from there. Find or make a lever, and place a fulcrum underneath it, off-center. Place a ball (or something else lightweight and soft) on the long end. Drop something heavy on the short end. Watch it fly!

## Patient Ping Pong Ball

A ball will sit patiently upon the half-circle at the end of a track. If you are making your own track from cardboard, use scissors, or better yet, a large hole-puncher if you have one, to make the half-circle.

The forces of gravity and friction keep the ball in place. That’s Newton’s first law – the law of inertia. That means that a body (an object) at rest tends to stay at rest, and a body in motion tends to stay in motion – with the same speed and in the same direction, until some other force makes it change. Like a kid who doesn’t want to get out of bed in the morning (body at rest), and then insists on staying up late at night (body in motion). Whatever force the parent exerts needs to be greater than the force keeping that kid asleep, or awake!

We have stored potential energy. The patient ping-pong ball waits patiently till some force or object transforms its potential energy into kinetic (movement) energy. And so, I am reminded to be patient with myself, especially when I am failing, which happens a lot, and in failing again and again, I am storing more and more of my own potential energy for me to release in my quest to understand how the world works, and to help make it work even better.

All complicated machines are made from simpler parts. If you haven’t mastered this technique, or don’t yet understand it, break it down into smaller parts till you “get” each one, and then build up from there. Find or make a track with a half-circle at the end. Lean the track to make a ramp. Place a ball gently upon the half-circle. Change the size of the half-circle if needed. Let one ball tap the next. Repeat.

## Gather Household Materials

#### Suggested Materials List

Surfaces: table, floor, shelf, counter, flat board,

Tools: drill, ruler, scissors, hot glue gun, hole punchers,

Toys: Hot Wheels Tracks, Tinker Toys, K’Nex, Legos, dominoes, popsicle sticks, railroad tracks, marble runs, action figures,

Tracks: cardboard, stretched rubber bands, booktops,

Balls: marble, ping pong ball, golf ball, rubber ball, rolled up aluminum foil, baseball, tennis ball,

Building Blocks: blocks, scrap wood, cans, boxes, bins, chairs, stools,

Tubes: Paper towel rolls, toilet paper rolls

Fasteners: rubber bands, glue, hot glue, tape, brass fasteners, string, wire (from clothes hangars), clamps, clothespins, twist ties, pipe cleaners, magnets,

Pullers: string, chain (lightweight), ribbon, floss, fishing line, shoelace, tape,

Rollers: balls, plastic containers, toy wheels, toy cars, fruits & vegetables, cans, tubes, dowels, cone shapes, lids, tape rolls, tape roll inserts, nuts & bolts, discs, paper wheels, spools,

Dominoes: dominoes, clothespins, CD cases, cassette tape cases, books, folded playing cards,

Weights: dominoes, blocks, coins,

Spinners: blocks with holes (bushings), rods, pen and marker housings and caps, things that turn freely like swivels (lazy Susans),