Thursday

Raven lays a new table on the patio. Each row names an activity or a machine and gives a percent.

"Cycling and climbing, 20 percent," she reads. "Swimming at the surface, 2 percent. What do you notice?"

Rocket frowns. "Twenty percent of what? Where is the other eighty?"

Nova hovers over the column. "Think about yesterday's rug," she says. "Where did the missing bounce go?"

"Warmth and sound," Rocket says slowly. "So a cyclist turns only a fifth of their food energy into motion?"

"And the rest warms the cyclist," Raven says. "That is why you sweat."

Rocket looks at the swimming row. "Two percent. Water takes almost everything."

Nova hums. "Now read the machines too."

Even though energy is conserved, not all of it ends up where you want it. The share that becomes useful work is called efficiency.

An OpenStax physics text defines efficiency as the useful energy or work output divided by the total energy input. It is written as a percent.

The text explains with a coal power plant. About 40 percent of the chemical energy becomes useful electrical energy. The other 60 percent becomes other forms, such as thermal energy.

Here are rows from the text's table of efficiencies for the human body and some machines.

Every row is an energy inventory. The percent is the useful part. The rest, 100 minus the percent, is energy that changed into other forms.

For the human body, most of the rest becomes thermal energy. That is why exercise makes you warm and why you sweat.

Swimming scores lowest. Pushing through water makes a lot of drag, and drag turns motion energy into warmth in the water.

A gasoline engine turns 30 percent into motion, so about 70 percent becomes other forms, mostly warmth. Engines get hot for that reason.

Your bounce test was an efficiency test too. A 72 percent bounce means 28 percent of the energy changed form.

Excellent data work. Tomorrow you will find energy changing form all over sport, and review the week.