Newton's Third Law
Forces and Motion

Describing interactions

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Modelling interactions with interaction diagrams.

This is a theoretical activity that provides a full framework for analysing interactions. It's worth doing a few times, so that the pattern becomes established.

What to Prepare

  • images and diagrams of a situation
  • a set of cardboard arrows to represent forces and labels (see below)
  • some coloured thread to pair up the forces

What Happens During this Activity

The key to this activity is to go slowly enough, and to be explicit enough, that the process gets internalised.

Teacher: Here we're moving from a situation where things interact to a description where we have pairs of objects, each in its own environment.

Here are the steps:

  • Identify the things that you intend to focus on.
  • Redraw the situation, so that the objects are brought to the foreground.
  • Identify the interactions of the objects with their environment.
  • Draw linked pairs of forces, one on the environment and one on the object, for each interaction.
  • Redraw each object, isolated from its environment but with the forces that represent the interactions.

Now you'll recognise that each object is ready for you to find the resultant force and apply Newton's second law.

But the process is very important, showing students how to use Newton's third law. That's the part where the forces appear in pairs: one on the object and one on the environment.


Download the support sheet / student worksheet for this activity.

Newton's Third Law
is used in analyses relating to Collisions
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