Newton's Laws · Grade 11

If there is no net force, why does a body keep moving?

Because motion needs no cause. Newton's first law says that a body with zero net force stays at rest if it was at rest, and carries on in a straight line at constant speed if it was already moving. What requires a force is a change in motion — slowing, speeding up or turning — not the motion itself.

Learning objectives

An inversion of intuition

Everyday experience says that keeping something moving takes a continuous push. A trolley you stop pushing stops, so motion looks as though it consumes force.

What is hidden is the friction. The trolley stops because the floor exerts a backward force on it, not because the motion 'ran out'. Remove the friction and it carries on.

That is why a spacecraft with its engines off does not slow down. There is no friction in space, the net force on it is zero, and it continues at the same speed in the same direction indefinitely. This is not a special case — it is the universe's default.

Equilibrium, and what it lets you extract

The condition is simple: the forces sum to zero. Not 'the body is at rest' but the forces sum to zero, and being at rest is only one of the two possible outcomes.

The second outcome is just as useful: a body moving in a straight line at constant speed is also in equilibrium. A car travelling at a steady 90 km/h is in exactly the same physical state as a parked one.

From that comes a method: when a body is at rest or at constant velocity, any unknown force follows from demanding that the sum be zero. That is why so many problems are solved without knowing anything about acceleration.

What a passenger feels when a bus brakes

When the bus brakes the passenger is thrown forward — that is how it feels. But nothing pushes them forward, and there is no body you could point to as exerting that force.

What happened is the opposite: a braking force acted on the bus, and none acted on the passenger. The passenger simply carried on at their previous speed — exactly as the first law says — while the vehicle around them slowed.

The sensation of 'a force pushing forward' is the habit of comparing ourselves to the vehicle rather than to the road. The seatbelt is what finally exerts a backward force on the passenger, and that is why it exists.

Worked examples

  1. A 2 kg lamp hangs at rest from the ceiling. What is the tension in the cord?

    1. The lamp is at rest, so the forces sum to zero
    2. Its weight is 2 times 10, so 20 N down
    3. The tension must balance it

    Answer: 20 N upwards

  2. A car moves at constant speed. The engine provides 800 N forward. What is the resistive force?

    1. Constant velocity means equilibrium
    2. The forces along the direction of travel must sum to zero

    Answer: 800 N backwards, exactly matching the engine

  3. A spacecraft in deep space shuts off its engines. What happens to its speed?

    1. There is no friction and nothing touching it
    2. The net force is zero

    Answer: It continues at the same speed and direction indefinitely

Common mistakes

Thinking motion requires a force
Motion needs no cause; only a change in it does. A trolley stops because of friction, not because the motion ended by itself.
Inventing a force that throws the passenger forward
No body exerts it. The passenger carries on at their previous speed while the vehicle slows, which is precisely what the first law predicts.
Equating equilibrium with being at rest
Constant velocity in a straight line is equilibrium just as much. A car at a steady 90 km/h is in the same physical state as a parked one.

What to remember

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