Aims of this podcast episode:

  • Understand what Newton’s three laws are;
  • Describe how they can be applied to different examples;
  • Explain the differences between each of the laws.

This can be a pretty complicated topic (if you let it), so we’ll start simply by clarifying some of the language often used when describing Newton’s laws.

Velocity: velocity is a combination of speed and direction, so it is often stated with a + or – sign (forwards or backwards). If something has a constant velocity, it has a steady speed and its direction is not changing.

Acceleration: acceleration is a change in velocity. Given the explanation of velocity above, you can accelerate by either changing your speed or changing your direction (or both); most of the time it’s just a change of speed.

Dimension: a dimension is a direction in space. Most of the time at GCSE we refer to two: up and down, left and right.

Resultant Force: this is the overall force acting in a certain dimension. For example if an object has a 10 N force pushing forwards and an 8 N force pushing backwards then the resultant force will be 10 – 8 = 2 N forwards.

Now, let’s begin by simply stating each of Newton’s three laws, we’ll then break them down. Having read this and listened to the podcast you should be a bit more confident in using Newton’s Laws of Motion.

Newton’s First Law (N1L): “An object will remain at rest or a constant velocity, unless acted upon by a resultant force.”

Newton’s Second Law (N2L): “F = m x a, where F is the resultant force in a dimension, m is the mass of the object in kg and a is the acceleration caused by the resultant force.”

Newton’s Third Law (N3L): “If object A pushes on object B then object B will push back with an equally large force, but in the opposite direction.”


Join the club! See if this helps.

Think of N1L and N2L separately from N3L. N1L and N2L describe how a force will affect an object: N1L tells you if anything will happen at all and N2L will give you a number for how much acceleration will be caused.

N3L can then be used to explain why a particular force exists. Let’s take air resistance as an example: Air resistance is caused by the car (object A) pushing on the air particles (object B). The air particles then push back with an equal force in the opposite direction (air resistance).

In the episode we mentioned that there are different ways of explaining how an aeroplane flies, so if you’re interested here’s a good website from NASA to help.

S@S 001: Newton’s Laws of Motion
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