Action and reaction with a metre rule
Illustrating Newton's Third Law of Motion.
Apparatus and materials
Health & Safety and Technical notes
Arrange furniture and observers to minimise the possibility of injury if one side pulls harder than expected.
The teacher and a student (or two students) pull at either end of a metre rule.
1 This very simple demonstration is the basis for a discussion which will probably need much repetition, as students begin to understand the application of Newton’s Third Law.
With your and student pulling on the metre rule ask
- Which way am I pulling you?
- Which way are you pulling me?
- Is it possible for me to pull you without you pulling me?
These two forces do not cancel each other out and come to no pull at all. You are able to feel the student’s pull quite well and the student can feel your pull. Only one of the forces acts on the teacher and one on the student.
'You pulling on the student' and 'the student pulling on you' form a ‘Newton pair’ of forces.
2 If the student objects that there are three bodies involved (two people plus the metre rule), then dispense with the metre rule and pull each other directly.
If you do not wish to discuss the role of the meter rule as a connector, then explain that you pull the rule and the rule pulls you. A tension force is transmitted along the rule until, at the other end, the rule pulls the student and the student pulls the rule. Tension forces depend on the material being able to 'hold itself together', otherwise the forces between the atoms and molecules would part company and the rule would break.
3 The reason why neither you nor the student is accelerated is because of the frictional force acting on your shoes. This balances the pull - unless one of you is standing on ice where friction is not great enough to prevent motion.
4 Newton's Third Law concerns pairs of forces that act on two different bodies. Body A acts on body B, and body B acts on body A. The forces acting are equal in size but opposite in direction.
This experiment was safety-checked in March 2005