Rocket lines up on the driveway like a sprinter. "On your marks," he says to himself, and bursts forward.
Raven times him and laughs. "You pushed the ground backward so hard your shoe squeaked."
"That is the whole trick," Rocket says, out of breath. "Push back, go forward. Push down, go up."
Nova hovers beside the start line. "What about a swimmer? Or a goalkeeper catching a ball?"
Raven thinks. "The swimmer pushes water backward. The water pushes her forward."
"And when a ball lands in a glove," Rocket says, "the glove pushes the ball as hard as the ball pushes it."
Nova brightens. "Now you are seeing pairs everywhere. That is the point."
Every sport is built on pushes and push backs. A physics text says stubbing a toe or throwing a ball confirms the third law.
A runner pushes the track backward. The track pushes the runner forward. Grippy shoes let the runner push harder.
A swimmer pushes water backward with her hands and feet. The water pushes her forward.
When a ball lands in a glove, the glove pushes on the ball to stop it. The ball pushes on the glove with equal strength.
That is why a hard catch stings. The force on the glove is as big as the force on the ball.
| Sport moment | Push | Push back |
|---|---|---|
| Sprint start | feet push the track backward | track pushes the runner forward |
| Swimming stroke | hands push water backward | water pushes the swimmer forward |
| Jump | feet push the ground down | ground pushes the jumper up |
| Catch | glove pushes the ball to stop it | ball pushes the glove just as hard |
A collision is two objects meeting and pushing on each other for a short time. The pushes are a pair.
When a ball collides with another ball, each pushes the other with equal strength in opposite directions.
A physics text describes two objects of equal mass heading toward each other at equal speed. They stick together and come to rest.
In that kind of collision, most of the motion energy turns into thermal energy and sound. You hear the thud and the objects warm a little.
Next week you meet forces that act without touching at all. Week 12 brings collisions back when you design a landing pad.
| Week review | True or false? |
|---|---|
| Forces come in pairs that are equal in strength and opposite in direction. | ? |
| The two forces in a pair act on different objects. | ? |
| A rocket pushes on the air behind it to move. | ? |
| When two balls collide, the heavier ball pushes harder than the lighter ball. | ? |
| A swimmer pushes water backward and the water pushes her forward. | ? |
Strong week, physicist. Tomorrow is Game Day: a family rolling collision contest in one straight line.