Raven lays two printed pages on the picnic table. "NASA's three laws of motion, and a wagon from a physics text."
Rocket reads the wagon page. "Two children push a wagon with a third child inside. It moves at a steady speed."
"What do you notice about the forces listed?" Raven asks.
"Five of them!" Rocket says. "Two pushes, friction, weight and the ground pushing up. And the wagon still goes steady?"
Nova hovers over the page. "Steady speed. What does the first law say about the net force?"
"Zero," Rocket says slowly. "So all five add up to nothing."
Raven nods. "Five forces, perfectly balanced. Let us read them one at a time."
NASA Glenn Research Center publishes a review table of Newton's three laws of motion. Here it is in plain words.
This week is about the first law. The second law comes next week, and the third law comes in week 9.
| Law | What NASA says |
|---|---|
| First law (inertia) | An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force. |
| Second law (force) | The acceleration of an object depends on the mass of the object and the amount of force applied. |
| Third law (action and reaction) | Whenever one object exerts a force on another object, the second object exerts an equal and opposite force on the first. |
The OpenStax physics text describes two children pushing a third child in a wagon at a constant velocity.
The wagon and its rider are the system. Five external forces act on that system.
Because the wagon moves at a constant velocity, all of the forces must add up to zero.
| Force on the wagon system | Direction | Where it comes from |
|---|---|---|
| Push from the first child | forward | the first child's hands |
| Push from the second child | forward | the second child's hands |
| Friction | against the motion | the wheel axles and the ground |
| Weight | down | Earth's gravity |
| Normal force | up | the ground pushing back |
OpenStax points out that the wagon does not move up or down. So the normal force from the ground balances the weight.
Forward and back, the two pushes together must balance the friction. Otherwise the wagon would speed up or slow down.
Pairs of forces can balance in each direction separately. That is how five forces add to zero.
OpenStax also explains free body diagrams. The object is drawn as a point, and each force is an arrow.
The length of each arrow shows the strength of the force. Equal arrows pointing opposite ways cancel.
| Statement | True or false? |
|---|---|
| The wagon has five forces on it and still moves at constant velocity. | ? |
| In a force diagram, a longer arrow means a stronger force. | ? |
| The weight of the wagon points forward. | ? |
| Equal arrows pointing in opposite directions cancel. | ? |
Excellent reading. Tomorrow you will find balanced and unbalanced forces all over sport, and review the week.