Forces and Motion

Collisions with electrostatic forces

Practical Activity for 14-16 PRACTICAL PHYISCS


An illustration of a collision with invisible forces.

Apparatus and Materials

Health & Safety and Technical Notes

Do not add extra capacitance to the Van de Graaff generator.

Van de Graaff generator safety

Read our standard health & safety guidance

Coat the two ping-pong balls with Aquadag to make their surfaces conduct. Using the fine nylon thread, suspend them one diameter apart, preferably from the ceiling but in any case on threads as long as possible.


  1. Switch on the Van de Graaff generator and charge each of the balls by allowing them successively to touch its sphere.
  2. Study collision processes between the charged balls by drawing back one of the threads and allowing the balls to swing together. The 90° angle between paths after collision may be seen.
  3. You could also demonstrate collisions of the charged ping-pong ball with the Van de Graaff sphere.

Teaching Notes

Step 3 provides a useful introduction on the scattering of alpha particles in gold foil.

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