Rocket jumps off the bottom step onto the grass and lands with his knees bent, sinking low.
"Soft landing," he says. "Hardly felt it."
Raven has been watching. "What do you notice? You bent your knees and took longer to stop."
Nova circles overhead. "Where did your motion energy go?" she asks.
"Into my legs, slowly," Rocket says. "Muscles, a little warmth. Not one big jolt."
"A physics text says the same thing," Raven says. "Bend the legs or roll. Spread the stop over more time."
Rocket grins. "Like a landing pad. Soft things take energy slowly."
Nova hums. "Hold that idea. You will need it in week 12."
Every stop in sport is an energy change. A runner slowing, a ball caught, a jumper landing: motion energy becomes other forms.
A physics text explains that landing stiffly stops a body over a very short distance. That means a very large force.
The text says a much better way to cushion the shock is by bending the legs or rolling on the ground. This increases the time over which the force acts.
It adds that a kangaroo does the same thing. The shock of each hop is cushioned by the bending of its hind legs.
Soft mats, bent knees and crash pads all do one job. They let the motion energy change form slowly instead of all at once.
Catch a hard throw and your palm stings and warms. Some of the ball's kinetic energy became thermal energy in your hand.
Rub your hands together and they warm up. A physics text explains that rubbing surfaces makes their atoms vibrate, and that energy becomes heat.
A cyclist is only about 20 percent efficient. Most of the food energy used becomes warmth, which is why cyclists sweat on a cold day.
A ball rolling to a stop, a skater gliding to a halt and a bounce that falls short all tell one story. Energy changed form, and the total stayed the same.
| Sport moment | Motion energy becomes |
|---|---|
| A ball bounces lower than its drop | Thermal energy in the ball and floor, and sound |
| A rolled ball stops on grass | Thermal energy from friction |
| A jumper lands with bent knees | Work in the muscles and a little warmth, spread over time |
| A runner sprints and sweats | Mostly thermal energy, with about 20 percent as motion |
| Week review | True or false? |
|---|---|
| The total energy stays the same while it changes form. | ? |
| A dropped ball bounces back lower because some energy becomes thermal energy and sound. | ? |
| Friction turns kinetic energy into thermal energy. | ? |
| An activity that is 20 percent efficient turns 20 percent of its energy into warmth. | ? |
| Bending your knees on landing makes the stopping force larger. | ? |
Great week. Tomorrow is Game Day: a family bounce contest. Next week, action and reaction.