Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Head on collision between trolleys with magnets attached

Demonstration

'Invisible' collisions - collisions when objects do not 'touch'.

Apparatus and materials 

Dynamics trolleys, 2

Horseshoe magnets, Alnico, 2

Sellotape

0-Gauge railway track with trucks (OPTIONAL)

Health & Safety and Technical notes


Read our standard health & safety guidance

1 Fix the strong magnets onto the ends of the dynamics trolleys. The magnets are repelling each other. Use Sellotape to secure them satisfactorily.  

2 Alternatively, you could use trucks on a gauge-0 railway track or gliders on an air track.

 

Procedure


Two trolleys

Show collisions between these trolleys when they are pushed gently towards each other.

 

Teaching notes


1 The collision is 'silent' as the trolleys/trucks do not physically collide. When the magnets on the trolleys/trucks come in range their magnetic fields interact and the trolleys/trucks are repelled. Momentum is transferred to the magnetic field. 

2 You could say: 
 
This demonstration is important as a reminder that in collisions we do not need to have contact. Is there really any 'contact' at the atomic level? In fact all that happens is that forces rise steeply to big repulsions as the atoms move closer. The repulsions grow so large that, even though they have a very short time to act during the collision, they are able to bring the colliding bodies to a stop and push them apart again. There is always a distance of closest approach. This grows smaller and smaller as the collisions become more violent when colliding bodies are projected towards each other at higher and higher speeds.

Magnets in a collision

3 The attractions between atoms or molecules are short range electrical forces extending out a few molecular diameters at the most. (See the experiment Soap film; the tension remains constant for thin films.) There must also be repulsions or else solids and liquids would collapse under the attraction. These are very short range, a small fraction of an atomic diameter. 
 
We think of the repulsive forces as not appearing until one atom goes ‘smack’ against another in a collision. There is no real ‘smack’, only a very sudden slowing and stopping and starting away again of an atom in the strong force field of another atom’s repulsion. 

Hill in place for close approaches

Considering only one atom the equilibrium position in which it should settle down is:  

  • at point X where the two forces just balance (where the resultant force is zero) 
  • where the potential energy is minimum at point Z. 

Atoms or molecules exert forces

In practice the atom does not settle down but remains in oscillation about the equilibrium point.

 

Related experiments


Soap film

Collision on an air-track

Elastic collision of trolleys

Inelastic collision of trolleys

Constant and varying forces between trucks