4.Main Content
experiments
Action and reaction with a metre rule
Demonstration
Illustrating Newton’s Third Law of Motion.
Apparatus and materials
• Metre rule
Safety
Read our standard health & safety guidance
Procedure
The teacher and a pupil (or two pupils) pull at either end of a metre rule and a discussion is initiated about who is pulling whom.
Teaching notes
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 are a ‘Newton pair’ of forces.
2 It appears in simple form that the teacher is pulling the student and the student is pulling the teacher. Both forces appear at the same time. However the students might object that there are three bodies involved because of the metre rule. If so then the metre rule can be dispensed with and the teacher and student can pull each other directly. For those who wish to discuss the role of the metre rule as a connector then the teacher pulls the rule and the rule pulls the teacher. 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 snap.
3 The reason why the student or teacher does not accelerate towards each other is because of the frictional force acting on the shoes of each of them balances the pull unless one of them is standing on ice where the friction is not great enough to prevent them moving.