The first law: force and motion.

Step 1 of 5

A steady push

A cart stands on the track with a fan on top. In this lesson there is no friction at all: the wheels turn without any resistance. When the fan switches on it will push the cart with a constant force of 0.5 N, and it will not switch off. As the cart moves it leaves an orange mark on the track every half second.

Predict what will happen to its speed, and only then watch.

Step 2 of 5

When the fan switches off

A constant force does not give a constant speed: it gives an acceleration, a steady change of speed. So what happens when the force goes away?

The fan is on now. Wait for the cart to pick up some speed, and while it is moving slide Fan push down to 0 N. Look at the gaps between the marks the cart leaves after the fan stops. If you missed it, press Reset and try again.

Step 3 of 5

Forces that balance

Most bodies have several forces on them at once. Now the cart’s wheels are stiff, and the track holds it back with a friction force, against the way it would move. The fan pushes the cart with 0.3 N, and it does not move.

Add the forces, each with its direction, and you get the net force. Answer, then choose On under Show the forces to see every force acting on the cart, each as an arrow.

Step 4 of 5

Motion at a steady speed

The two cases, the cart that kept rolling after the fan stopped and the cart that stayed put against the push, are one law. This is Newton’s first law, the law of inertia:

When the net force on a body is zero, its velocity does not change. A body at rest stays at rest, and a moving body carries on in a straight line at the same speed.

Back to the experiment ↑

“The same velocity” includes the direction: turning needs a force too. A body’s tendency to keep doing what it is doing is called inertia. In everyday life things stop when you stop pushing them, because there is always friction or air resistance. A car cruising at a steady speed does not “need” a force to move: the engine only balances the forces that slow it down.

Try it: slowly raise Fan push. The cart stays put as long as friction can balance the push, and moves only once the push is bigger than that. You will come back to this limit in the lesson on friction.

Back to the experiment ↑

Step 5 of 5

In summary

• A force is a push or a pull, measured in newtons. The net force is all the forces on a body added up, with their directions.
• A net force changes the velocity. It is not needed to keep it.
• When the net force is zero, a body at rest stays at rest and a moving body carries on at the same speed in the same direction: Newton’s first law.
• A body standing still is not necessarily “free of forces”: the forces on it may balance.