# 5.7.1 Newton's First Law

### Newton's First Law of Motion

• Newton’s first 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 the resultant force acting on an object is zero:
• The object will remain stationary if it was stationary before
• The object will continue to move at the same velocity if it was moving
• If the object was moving, it will move at the same speed and in the same direction as before

### 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
• A few examples with uniform velocity are shown below:

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.

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

### Author: Jonathan

Jonathan graduated with a first-class Master's degree in Theoretical Physics from Imperial College London. He has worked in education for more than a decade as a Maths and Physics Teacher, Tutor, Head of Physics, and most recently, as Assistant Headteacher. He is now an Educational Consultant and works with us to design and improve our Physics resources.
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