Rocket dribbles a playground ball across the driveway. "Every bounce is a force," he says. "My hand, the ground, my hand."
Raven watches. "What do you notice when the ball is between your hand and the ground?"
"Nothing is touching it," Rocket says. "But it still moves. Inertia, right? Plus gravity pulling it down."
Nova circles them slowly. "And when you stand still holding the ball?"
"Balanced," Rocket says. "Gravity down, my hands up. The ball stays."
Raven writes it in the Field Log. "A whole week of physics in one bounce."
"And I did not even measure anything," Rocket says. Nova hums. "Next week you will."
A player standing still with a ball has balanced forces on the ball: gravity down, hands up. Nothing changes.
OpenStax describes a puck on an air hockey table. With the air on, friction is almost gone and its speed barely changes.
On a frictionless surface, the text says, the puck would feel no net external force and keep its speed.
In a tied tug of war, the rope's middle holds still. Two teams pull with equal and opposite force.
A kick, a throw, a swing or a catch is an unbalanced force. Each one changes the ball's speed or direction.
A ball rolling to a stop on grass is slowed by friction, an unbalanced force against its motion.
A dribbled ball falls because gravity pulls it down with nothing to balance it. Then the ground pushes it back up.
OpenStax says a change in motion is a change in speed, a change in direction, or both.
| Week review | True or false? |
|---|---|
| A force is a push or a pull. | ? |
| Balanced forces change an object's motion. | ? |
| An object in motion keeps its speed and direction unless an unbalanced force acts. | ? |
| Friction is an unbalanced force that slows a rolling ball. | ? |
| A change in direction without a change in speed is not a change in motion. | ? |
Great week of force spotting. Tomorrow is Game Day: a family tug of war with rules from physics.