Raven spreads a printed table on the patio table. Each row names two surfaces and gives two numbers.
"Shoes on wood, 0.9 and 0.7," she reads. "Shoes on ice, 0.1 and 0.05. What do you notice?"
Rocket scans the rows. "Ice numbers are tiny. Wood numbers are big. Bigger must mean more friction."
Nova dims her light so the small print is easier to read. "Why are there two numbers in every row?" she asks.
Rocket remembers the crate. "One for starting, one for sliding. Starting is harder."
"The first number is always bigger," Raven says, running her finger down. "Every single row."
"So the table agrees with Monday," Rocket says. Nova hums. "Now use it to predict something."
Physicists measure how much friction two surfaces make and write the result as a number called a coefficient of friction.
The number has no units, and it is usually between 0 and 1. A bigger number means more friction between those two materials.
Each pair of surfaces gets two numbers. The first is for static friction, before sliding starts. The second is for kinetic friction, while sliding.
Here are some rows from an OpenStax college physics text hosted by the University of California, Davis.
| Two surfaces touching | Static friction | Kinetic friction |
|---|---|---|
| Rubber on dry concrete | 1.0 | 0.7 |
| Rubber on wet concrete | 0.7 | 0.5 |
| Shoes on wood | 0.9 | 0.7 |
| Wood on wood | 0.5 | 0.3 |
| Waxed wood on wet snow | 0.14 | 0.1 |
| Shoes on ice | 0.1 | 0.05 |
| Ice on ice | 0.1 | 0.03 |
| Steel on ice | 0.04 | 0.02 |
The friction between two surfaces depends on both materials. Shoes on wood and shoes on ice have the same shoes but very different numbers.
Steel on ice is 0.02 when sliding, the smallest in the table. That is why an ice skate glides so far on a rink.
Waxed wood on wet snow is 0.1 when sliding. A waxed ski slides easily, which is the whole point of waxing it.
Rubber on dry concrete is the biggest number. Rubber soles grip a dry court, and a rubber ball on a patio stops fast.
Your Field floors are not in this table, but the idea is the same. The surface that stopped your ball soonest has the bigger number.
| Statement | True or false? |
|---|---|
| A bigger coefficient of friction means more friction. | ? |
| Shoes have the same friction on every surface. | ? |
| Steel on ice has less kinetic friction than wood on wood. | ? |
| The coefficient of friction is measured in metres. | ? |
Excellent data work. Tomorrow you will find friction and drag in sports, and review the week.