Thursday

Raven spreads a table across the patio: ten worlds and a column of gravity numbers.

"Earth, 9.8. Moon, 1.6. Jupiter, 23.1," she reads. "What do you notice?"

"Jupiter pulls more than twice as hard as Earth," Rocket says. "And the Moon barely pulls."

Nova hovers over the Moon column. "You lift the same ball one metre on each world," she says. "Where is the lift hardest?"

"Jupiter," Rocket says. "The ball weighs the most there."

"And the harder the lift, the more energy you store," Raven says. "Same height, different arrangement."

Rocket grins. "So a one metre shelf on Jupiter is a bigger deal than a one metre shelf on the Moon."

Nova hums. "Now read the whole table."

NASA publishes a planetary fact sheet with the surface gravity of each world in metres per second squared.

Gravity here means how fast a dropped object speeds up near the surface. A bigger number means a stronger pull.

Earth's number is 9.8. A NASA guide says the weight of an object is its mass times this gravity number.

Here are the gravity values from the fact sheet.

The physics text works out the energy stored by a lift. The force needed to lift an object at a steady speed equals its weight.

Weight is mass times gravity, so the same ball weighs more where gravity is stronger. Lifting it one metre there stores more energy.

The text gives an Earth example. A half kilogram mass raised one metre stores about 4.9 joules.

Earth's gravity, 9.8, is about six times the Moon's 1.6. So the same lift on Earth stores about six times the energy it would on the Moon.

This is the standard's big idea. Change the arrangement of two objects that attract each other, and the stored energy changes.

Excellent data work. Tomorrow you will find stored energy in sports and review the week.