Newton's First Law (AQA GCSE Physics)

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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 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:

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.

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

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.