Rocket holds a tennis ball at arm height. "I will drop it, and you tell me if it speeds up," he says.
Raven watches it fall. "Too fast to see. What do you notice about the sound, though? One sharp bounce."
Rocket drops it from his knee. The bounce is softer. "Lower drop, slower landing," he says.
Nova hovers near the landing spot. "So the longer it falls, the faster it goes. What does that tell you about gravity?"
"It keeps pulling," Rocket says. "It does not give the ball one shove. It pulls the whole way down."
Raven writes in the Field Log. "A steady pull makes a steady speed-up."
"Then a feather should do the same," Rocket says. Nova hums. "On the Moon, it did."
A university physics text explains that gravity causes objects to fall toward the center of Earth.
If air resistance is negligible, every object at a given place falls with the same constant acceleration, whatever its mass.
The text gives Earth's value as about 9.80 metres per second each second. A falling ball gains about 9.8 metres per second of speed every second.
The direction of this acceleration is downward, toward the center of Earth. Its direction defines what we call vertical.
If the object is dropped, it starts with zero speed. Once it leaves your hand, it is in free fall.
| Seconds since the drop | What the ball is doing |
|---|---|
| 0 | leaves the hand at zero speed |
| a little later | moving down, picking up speed |
| later still | moving down faster |
| until it lands | still speeding up the whole way |
The same text calls it the most remarkable fact about falling objects. Without air resistance, all objects fall with the same acceleration.
It is unexpected, the text says, because we are used to air resistance and expect light things to fall slower.
In 1971, astronaut David R. Scott dropped a hammer and a feather together on the Moon, where there is no air.
They fell together. The Moon's acceleration due to gravity is only 1.67 metres per second each second, the text notes.
On Earth, the text adds, air resistance can make a lighter object of the same size fall slower than a heavier one.
| Statement | True or false? |
|---|---|
| Without air resistance, a heavy ball and a light ball fall together. | ? |
| A dropped ball starts with zero speed. | ? |
| Gravity gives a falling ball one shove and then stops pulling. | ? |
| The hammer and feather test was done on the Moon in 1971. | ? |
Careful thinking. Tomorrow is Field Lab: you will drop and time two objects from two heights.