Newton and his 3rd Law

A quick comment about this law – arguably the most complex of the three laws.

My experience is that teachers often present Newton’s Laws in the way the books normally state them. This is something along the lines of …

Newton’s 1: An object will move in a straight line at constant speed unless acted on by an external force.

Newton’s 2: If an external unbalanced force acts upon an object, the unbalanced force will produce a rate of change of momentum and the direction of the unbalanced force is in the direction of the rate of change of momentum.

Newton’s 3: For every action, there is an equal but opposite reaction.

Wow – the first and second laws could be stated more concisely and the third law creates ambiguity, and would actually benefit (at least with respect to understanding) from being much longer.

When I present these laws, I ensure that initially, students know what the term acceleration means. For most students, acceleration is a reference to being in a car and the car is speeding up. I have to go back to the basic definition and remind them that a is the rate of change of momentum. We then have a discussion albeit brief, on what the word rate means. Then to emphasise, I give them a sentence I frequently ask them to repeat to me …

“Acceleration means speeding up, slowing down or changing direction”

(I actually introduce this sentence to them early in the GCSE years – it becomes almost like a mantra).

I then discuss Newton’s laws with respect to what they are actually about … forces:

Newton’s 1: This is about the motion of an object when the forces on it are balanced …
“If the forces on an object are balanced, it will not accelerate i.e., a=0”

Newton’s 2: This is about the motion of an object when the forces on it are unbalanced …
“If the forces on an object are unbalanced it will accelerate i.e., a ≠0”

Newton’s 3: This is about forces in pairs.
“For every force, there is an equal but opposite force”

I have found that if I now get students to think about Newton’s first and second law above and compare them to the sentences in their textbook, the ensuing discussion opens their eyes a little more. However … I still have the issue of newton’s third law.

An example of the problem with this law is shown with a calculator sitting on a desk. Ignoring buoyancy, there are two forces and they are equal but opposite. And students often think that this is an example of N3. To help overcome the problem, I explain that N3 has been shortened and should be much bigger …

“For every force acting on an object there is an equal but opposite force of the same type, and this equal but opposite force is acting on a different object”

We then have a conversation about this overly long sentence. A final conclusion is that when you look at an object and identify the actual forces acting on it, there is an equal but opposite force for every actual force.

I have found that the above focus on getting the laws better-understood, works for students who are 14y and above.