Newton's Laws of Motion

Newton's First Law: the law of inertia

An object stays at rest, or keeps moving at a constant velocity, unless a net force acts on it.

Objects "like" to keep doing what they are already doing. This resistance to change is called inertia. The more mass an object has, the more inertia it has. That is why a loaded truck is harder to stop than a bicycle.

Everyday examples

• Ella lurching forward when the bus brakes.
• A tablecloth pulled away fast, leaving the plates behind.
• Seatbelts: they provide the force to stop you when the car stops suddenly.

Newton's Second Law: F = ma

The acceleration of an object depends on the net force on it and its mass.

$$F_{\text{net}}=m\times a$$

• $F$ = net force (N)
• $m$ = mass (kg)
• $a$ = acceleration (m/s²)

So 1 newton is the force that makes a 1 kg mass accelerate at 1 m/s².

What this means:

• More force gives more acceleration (same mass).
• More mass gives less acceleration (same force).

Newton's Third Law: action and reaction

When object A pushes on object B, object B pushes back on object A with a force that is equal in size and opposite in direction.

Forces always come in pairs.

• You push down on the floor. The floor pushes up on you.
• A swimmer pushes water backwards. The water pushes the swimmer forwards.
• A rocket pushes hot gas down. The gas pushes the rocket up.
• Earth pulls you down. You pull Earth up with exactly the same force.

Quick summary

• First Law: no net force means no change in motion. This is inertia. More mass, more inertia.
• Second Law: $F_{\text{net}}=ma$. Bigger force, bigger acceleration. Bigger mass, smaller acceleration.
• Third Law: forces come in equal and opposite pairs acting on different objects.
• Always use the net force in $F=ma$.
• 1 N accelerates 1 kg at 1 m/s².

Quiz