Raven lays out a new table on the patio: objects, masses, speeds and a column of joules.
"A 30 kilogram package at half a metre per second, under 4 joules," she reads. "A basketball at 7.5 metres per second, 17.6 joules."
Rocket frowns. "The package weighs fifty times more and has less energy? That cannot be right."
Nova hovers over the speed column. "What do you notice about the speeds?" she asks.
"The package is crawling," Rocket says slowly. "The ball is flying. And speed counts extra."
"Now look at the sprinter," Raven says. "80 kilograms at 10 metres per second: 4,000 joules."
Rocket whistles. "Heavy and fast. That is the big number." Nova hums. "Read the rest and find the pattern."
Physicists work out kinetic energy from mass and speed. Here are worked examples from two OpenStax physics texts hosted by LibreTexts.
Mass is in kilograms and speed is in metres per second, the units you learned in week 1. Energy is in joules.
Read the mass and speed first, then the energy. Look for which column changes the energy more.
| Moving object | Mass (kilograms) | Speed (metres per second) | Kinetic energy (joules) |
|---|---|---|---|
| Package on a conveyor belt | 30 | 0.5 | 3.75 |
| Basketball in a pass | 0.624 | 7.5 | 17.6 |
| Person walking | 75 | 1.5 | 84.4 |
| Athlete running | 80 | 10 | 4,000 |
| Person moving with a train | 75 | 13.5 | 6,830 |
| Person moving with a faster train | 75 | 16.5 | 10,200 |
The person at 13.5 metres per second has 6,830 joules. At 16.5 metres per second, the same person has 10,200 joules.
The speed went up by less than a quarter, but the energy went up by about a half. Speed counts extra.
The athlete running at 10 metres per second has about 47 times the energy of the walker at 1.5 metres per second.
Another physics table lists a tennis ball at 100 kilometres per hour with about 22 joules. A light ball at high speed still carries real energy.
This is why a hard throw can hurt and a gentle roll cannot. Soft balls and gentle rolls keep the Field safe.
| Statement | True or false? |
|---|---|
| The heaviest object in the table has the most kinetic energy. | ? |
| At the same mass, the faster person has more kinetic energy. | ? |
| The basketball has more kinetic energy than the slow package. | ? |
| Kinetic energy in the table is measured in kilograms. | ? |
Excellent data work. Tomorrow you will find kinetic energy in sports and review the week.