Newton's First Law

Exam code: 5054

Written by: Ashika|Reviewed by: Caroline Carroll|Updated 2 July 2026

Newton's First Lawvideo

Newton's First Law

Newton's First Law of Motion

Newton's 1st law of motion states:

Objects will remain at rest, or move with a constant velocity unless acted on by a resultant force

This means if that:
- A stationary object will remain stationary
- An object moving with a constant velocity will continue to move at that constant velocity
- Unless the object is acted upon by a resultant force
Conversely, that means that if an object is at rest, or has a constant velocity, no resultant force can be acting on it
Remember that a constant velocity means:
- No change in speed
- No change in direction

Applying Newton's First Law

Newton's first law is used to explain why things move with a constant (or uniform) velocity
- If the forces acting on an object are balanced, then the resultant force is zero
- The velocity (i.e. speed and direction) can only change if a resultant force acts on the object

Examples of Objects with Uniform Velocity

Newtons First Law in Action, downloadable IGCSE & GCSE Physics revision notes

Constant velocity can only be achieved when the forces on an object are balanced - in other words, when the resultant force is zero

Worked example

Lima did some online research and found out that the Moon orbits the Earth at a constant speed of around 2000 mph. She says that this is not an example of Newton's first law of motion.

Is Lima correct? Explain your answer.

Answer:

Step 1: Recall Newton's first law of motion

Newton's first law of motion states that objects will remain at rest, or move with a constant velocity, unless acted on by a resultant force

Step 2: Determine if the object in the question is at rest, or if it is moving with a constant velocity

The Moon, in this case, is not at rest
It is moving at a constant speed
But it is not moving in a constant direction - it continually orbits the Earth
Hence, it is not moving with a constant velocity, because velocity is a vector quantity

Step 3: State and explain whether Lima is correct

Lima is correct
The Moon moves with a constant speed, but always changes direction
So it is not moving with a constant velocity, and is not an example of Newton's first law of motion

Exam tip

Students often struggle to understand how an object can move when there is no resultant force acting on it. You may even have this misconception, without realising it.

It is important to fully understand Newton's first law, because it underpins a lot of physics content and can cause you to make mistakes in seemingly unrelated questions.

Usually, students are comfortable with the idea that an object at rest on a table will not move unless something or someone physically moves it. The issue comes from the idea of an object in motion.

A rock drifting through the vacuum of deep space away from the gravitational pull of any large bodies, will continue to drift at that same speed, in that same direction, potentially forever. The only way to change its motion is for a force to be exerted upon it.

On Earth, there are always forces acting on objects, so in our everyday experience we need to think in terms of resultant forces and balanced forces.

A car can maintain a constant velocity when the force from the engine is balanced with the frictional forces opposing its motion (friction between the tyres and road, and air resistance). If nothing changes with these forces, it will continue travelling at a constant velocity. If you do want to change the motion of the car, to speed up or slow down, you need to provide a resultant force. By increasing the force from the engine, you can cause the car to speed up. By either decreasing the force from the engine, or increasing the frictional force (using the brakes) the car will slow down.

When the forces acting on an object are not balanced (a resultant force is exerted) this will cause a change in the object's motion.

Newton's First Lawvideo

Newton's First Law of Motion

Applying Newton's First Law

Examples of Objects with Uniform Velocity

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