Raven draws two columns in the Field Log: Force and Mass. "Two things decide how a motion changes," she says.
Rocket taps the first column. "Bigger push, bigger change. I knew that one."
"And the second?" Nova asks, hovering over the page.
"Bigger mass, smaller change," Rocket says. "The heavy ball yesterday."
Raven nods. "So what if you want the heavy ball to speed up as much as the foam ball?"
Rocket thinks. "Push it harder. A lot harder."
"What do you notice?" Raven asks. "You just said Newton's second law in your own words."
NASA Glenn Research Center states the second law this way. The acceleration of an object depends on its mass and the amount of force applied.
Acceleration is the physics word for a change in motion: speeding up, slowing down or turning.
NASA adds that for equal forces, a heavier object will experience less acceleration than a lighter object.
OpenStax says the same from the other side. To speed up a more massive object by the same amount, more force is needed.
The Next Generation Science Standards sum it up. For any given object, a larger force causes a larger change in motion.
| Keep the same | Change this | What happens to the change in motion |
|---|---|---|
| the ball | push harder | a bigger change |
| the ball | push more gently | a smaller change |
| the push | use a ball with more mass | a smaller change |
| the push | use a ball with less mass | a bigger change |
OpenStax explains that the change in motion is directly proportional to the net force.
Directly proportional means that if one doubles, the other doubles too.
The change in motion is inversely proportional to the mass. If the mass is multiplied by a number, the change is divided by that number.
So doubling the mass, with the same push, cuts the change in motion in half.
OpenStax pictures a boy pushing a basketball, then pushing a stalled car with the same force. The ball speeds up far more.
| Same push on... | Mass compared with the foam ball | Change in motion compared with the foam ball |
|---|---|---|
| the foam ball | the same | the same |
| a ball with twice the mass | two times | one half |
| a ball with three times the mass | three times | one third |
| Statement | True or false? |
|---|---|
| The change in motion depends on both the force and the mass. | ? |
| For equal forces, a heavier object speeds up more than a lighter one. | ? |
| Acceleration means a change in motion. | ? |
| Tripling the mass with the same push cuts the change to one third. | ? |
Strong reasoning. Tomorrow is Field Lab: a ramp gives the same push to two balls of different mass.