Rocket lines up his shoes on the patio: rubber soled sneakers, smooth soled dress shoes and a pair of socks.
"Which would you wear to run on the court?" he asks, already knowing.
Raven points at the sneakers. "Rubber grips. What do you notice about the dress shoes? Smooth bottoms, like wood on wood."
Nova circles above the line of shoes. "Friction helped yesterday's ball stop," she says. "When does an athlete want more of it?"
"Starting, stopping, turning," Rocket says. "And when does an athlete want less?"
Raven thinks of the friction table. "Skating. Skiing. Sliding into a base."
"And swimmers and cyclists want less drag," Rocket adds. Nova brightens. "Then you are ready to review."
A rubber sole slips less than a leather sole. The physics text explains that friction depends on the materials that touch, not just on roughness.
Friction between shoes and the ground is what lets you start, stop and turn. Without it you could not walk, as anyone who has crossed ice knows.
Rubber on dry concrete has the biggest number in yesterday's table. That grip is why court shoes have rubber soles.
Sometimes an athlete wants less friction. A skate on ice and a waxed ski on snow have tiny numbers, so they glide.
Friction also uses up energy of motion. When surfaces rub, they warm up. You will measure that in week 8.
Athletes and car designers work to lower drag. Less drag means less force pushing back, which means faster times.
A smooth, streamlined shape has less drag than a blunt one. The physics text lists a thin wing shape at 0.05 and a flat round plate at 1.12.
A sphere, the shape of a ball, sits in the middle at 0.45. Cyclists and swimmers wear smooth suits to cut drag, and some shave their body hair.
A skydiver falling feet first has less drag than one spread flat. Smaller face to the air, less drag, faster terminal velocity.
| Shape | Drag coefficient |
|---|---|
| Thin wing shape (airfoil) | 0.05 |
| Sphere (a ball) | 0.45 |
| Skydiver, feet first | 0.70 |
| Bicycle | 0.90 |
| Skydiver, spread flat | 1.0 |
| Flat round plate | 1.12 |
| Week review | True or false? |
|---|---|
| Friction opposes motion between surfaces that touch. | ? |
| A ball rolls farther on carpet than on a smooth floor. | ? |
| Drag grows as an object moves faster. | ? |
| A flat sheet of paper has more drag than a crumpled one. | ? |
| At terminal velocity the forces on a falling object are unbalanced. | ? |
Great week. Tomorrow is Game Day: your family drops paper filters to find terminal velocity. Next week, kinetic energy.