Newton's Second Law
Forces and Motion

Apply those brakes

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Making predictions.

This is an exploration that gives reliable, valid results, which can be a good way of considering how assertions are related to the evidence.

What to Prepare

Per group:

  • a flexible track to run a cheap model car down
  • an adapted model car
  • an old pen case, with a rubber fixed in the end
  • six 5 gram masses
  • a small object to trigger the brakes
  • a metre ruler
  • the interactive object (see below), displayed on a large screen

What Happens During this Activity

Introduce the apparatus and show how the brakes are applied when the car runs past the small obstruction. Show how the braking force can be changed by adding masses to the top of the brakes. Ask how to measure the braking distance (from where the brakes are applied to where the car comes to a halt).

Ask how the pupils expect the braking distance to change if you double the force? Try and show them how this pattern will work out using a sketch graph – perhaps using the interactive provided. Then challenge them to find out if their expectations are correct by plotting a graph from their own data of braking distance against braking force.

Carefully done experiments very reliably give this shape of graph, confirming that the distance is inversely proportional to the force.

As an extension you can also measure how the distance depends on the energy in the kinetic store, simply by releasing the car from steadily increasing heights. Both of these results have interesting links to road safety. The first to driving in adverse conditions, the second to safety related speed limits.


Download the interactive for this activity.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law
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