Rocket sets a tennis ball on the driveway. It sits there. "Boring," he says. "Nothing is pushing it."
Nova dips toward the ball. "Is that true? What is pulling the ball down right now?"
"Gravity," Rocket admits. "But the ground holds it up. So they cancel."
Raven rolls the ball along the driveway. It slows and stops near the garage. "What do you notice?"
"It stopped by itself," Rocket says. Then he pauses. "Or did something stop it?"
"Something always does," Nova says. "A moving thing never changes its motion for no reason."
Rocket picks up the ball. "So a force started it and a force stopped it. Balanced in between?"
NASA Glenn Research Center states the rule. An object at rest remains at rest, and an object in motion remains in motion.
It keeps the same speed and the same straight line unless acted on by an unbalanced force.
NASA calls this tendency to resist changes in motion inertia. If all the forces cancel, there is no net force.
With no net force, the object keeps a constant velocity. For a ball at rest, that constant velocity is zero.
The Next Generation Science Standards say the same thing. If the total force on an object is not zero, its motion will change.
| Situation | Forces on the object | Balanced or unbalanced? | What happens to the motion |
|---|---|---|---|
| Ball resting on the driveway | gravity down, ground pushing up | balanced | stays at rest |
| Knot in a tied tug of war | two equal pulls | balanced | stays at rest |
| Ball rolling and slowing | friction against the motion | unbalanced | speed drops |
| Hand pushing a resting ball | push from the hand | unbalanced | starts to move |
OpenStax warns that the first law may seem to contradict everyday experience. A moving object usually slows down and stops.
The key is to see that a net external force acts to make it slow. That force is called friction.
Friction acts opposite to the velocity. Without it, a sliding box would keep sliding at a constant velocity.
OpenStax describes an air hockey table. With the air off, the puck slides a short way and stops.
With the air on, the puck is lifted slightly and feels very little friction. It glides with very little change in speed.
| Statement | True or false? |
|---|---|
| A ball at rest has no forces on it at all. | ? |
| Balanced forces leave an object's motion unchanged. | ? |
| A moving ball needs a steady push to keep moving at constant speed on a frictionless surface. | ? |
| Friction acts opposite to the direction of motion. | ? |
Clear thinking. Tomorrow is Field Lab: you will tug, push and pull a backpack to see balanced and unbalanced forces in action.