Monday
Monday: Energy on a Shelf
Raven sets a foam ball on the top of a tall cardboard ramp propped on a stack of books on the patio.
Beside it, a second ramp made of one book waits with a matching ball.
"Two balls, both sitting still," she says. "Zero kinetic energy each. What do you notice?"
Rocket is stretching a rubber band between his fingers and grinning. "Mine is sitting still too. But it wants to go."
Nova hovers above the taller ramp. "If both balls have no motion energy, why does the high one worry you more?" she asks.
"Because it has farther to fall," Rocket says. "It is loaded. Like my rubber band."
Raven lets the high ball go. It races down and crosses the patio. The low ball barely makes it off the book.
"Energy was waiting," Rocket says. Nova hums. "Stored energy has a name."
Potential energy: stored and recoverable
A ball sitting still has no kinetic energy. But a ball on a high shelf can still do something a ball on the floor cannot. It can fall.
A physics text defines potential energy as the energy a system has due to position, shape or configuration. It is stored energy that is completely recoverable.
Recoverable means you can get it back. Let the ball go, and the stored energy turns into motion.
Lifting a ball stores gravitational potential energy. The higher you lift it, the more is stored.
Stretching a rubber band or bending a ruler stores elastic potential energy. The more you stretch it, the more waits inside.
| Kind of potential energy | How you store it | Field example |
|---|---|---|
| Gravitational | Lift an object higher | A ball at the top of a tall ramp |
| Elastic | Stretch, squeeze or bend something springy | A stretched rubber band or a bent ruler |
- Read the question.
- Tap your answer.
Why the energy belongs to a system
The physics text says potential energy is a property of a system, not of a single object. The ball and Earth together store it.
Gravity pulls the ball and Earth toward each other. Lifting the ball apart from Earth stores energy in that separation.
The science standards say it this way: a system of objects may contain stored energy, depending on their relative positions.
Change the arrangement, change the stored energy. Lift the ball higher and more is stored. Lower it and less is stored.
The standards give the examples: a roller coaster cart at different positions on a hill, or objects on shelves at different heights.
| Statement | True or false? |
|---|---|
| Potential energy is stored energy that can be recovered. | ? |
| A ball on a high shelf has more gravitational potential energy than the same ball on the floor. | ? |
| A stretched rubber band stores kinetic energy. | ? |
| Potential energy belongs to one object by itself. | ? |
Stretch a rubber band a little, then more. Feel the pull grow in your fingers. Point it at the ground, never at anyone.
Write "stored" beside each one in your Field Log and say where the energy came from.
Rubber bands are stretched toward the ground and never aimed at a person or pet. Ramps sit on the floor or a low step.
The potential energy facts come from an OpenStax college physics text hosted by LibreTexts and from the Next Generation Science Standards.
Good start. Tomorrow you will follow stored energy as it turns into motion, and back again.