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Newton's First Law of Motion

Classical Mechanics

Newton's first law of motion is often called the law of inertia. It tells us that an object will keep doing what it is already doing, staying still or moving steadily, unless a net force pushes or pulls it into a change. Once you understand inertia, many everyday sights, from a sliding hockey puck to a passenger lurching in a braking bus, start to make sense.

What the law says

Newton's first law states that an object at rest stays at rest, and an object moving in a straight line at a constant speed keeps moving that way, unless a net force acts on it. The key word is net force: the total of all the forces acting on the object. If the forces balance out, the net force is zero and there is no change in motion. This tendency to resist a change in motion is called inertia, and it is a property of all matter. The more mass an object has, the more inertia it has, so the harder it is to start, stop, or turn.

Inertia in everyday life

You feel inertia every time a car starts or stops. When a car accelerates forward, your body seems to press back into the seat because it tends to stay where it was. When the car brakes sharply, you lurch forward because your body tends to keep moving. A tablecloth can be pulled quickly from under dishes because the dishes' inertia keeps them almost in place. Seat belts and airbags work with this idea too: they slow your body down gradually with the car instead of letting it keep moving forward on its own.

Why moving objects seem to stop

In everyday life, moving objects do seem to slow down and stop on their own, which can make the first law feel wrong. In fact, they are being slowed by unbalanced forces such as friction and air resistance. A ball rolling across grass stops because friction with the ground and drag from the air act against its motion. In space, where there is almost no friction or air, a spacecraft can coast for a very long time without its engines on. Remove the opposing forces, and the motion continues just as the law predicts.

Common mistakes

• Thinking a constant force is needed to keep an object moving at a steady speed. Correction: a steady speed needs zero net force; a force is only needed to overcome friction or to change the motion.
• Saying inertia is a force. Correction: inertia is not a force at all, it is the tendency of mass to resist a change in motion.
• Assuming heavier objects always fall or stop faster. Correction: inertia depends on mass, but how quickly something stops depends on the forces acting on it, not on inertia alone.

Practice questions

1. A book sits on a table and does not move. What is the net force on it?
   Show answer

   0 N. Gravity pulls down and the table pushes up with an equal force, so the forces balance and the net force is zero.

2. A spacecraft in deep space switches off its engines while travelling at 5000 m/s. What happens to its speed?
   Show answer

   It stays at 5000 m/s. With almost no friction or air in space, the net force is near zero, so there is no acceleration.

3. Why do you lurch forward when a bus brakes suddenly?
   Show answer

   Your body tends to keep moving forward at the bus's old speed (inertia). The bus slows down around you, so you move forward relative to it until the seat or a handrail stops you.

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