Forces on a student and skateboard are equal and opposite.
Apparatus and materials
Skateboards or demonstration trolleys, 2
Board, with one dimension larger than the skateboard wheelbase, with screw-eye fixed to it
Health & Safety and Technical notes
The activity should be done well away from hard edges such as those of lab furniture, with participating students wearing safety protection as recommended by skateboard manufacturers.
Use demonstration trolleys (either bought or home-made) or skateboards.
a A student walks, from in front or behind, towards a stationary skateboard. She/he steps on to the skateboard and stops walking.
b A student steps onto the trolley from the side, and then starts to walk forwards.
c A student walks towards a skateboard as in step a, but carries on walking.
d Place the board onto the ball bearings and the skateboard onto the board, with the screw-eye at the front. Fix the rope to the screw-eye. Pull the rope to exert a force on the board.
e Two students sit or stand, facing each other a few metres apart, on two skateboards. Each takes one end of the rope and both pull. Repeat this with one student pulling actively and the second merely responding. Then the second pulls while the first responds.
1 In step a, the student and skateboard experience equal and opposite forces. The skateboard experiences a forwards acceleration and the student experiences a backwards acceleration. The student's acceleration is much smaller because of his or her greater mass. The walker stops relative to the skateboard, she/he and the trolley together move forwards.
2 In step b, forces on student and skateboard are again equal and opposite. The student experiences positive acceleration while the skateboard experiences negative acceleration.
3 In step c, the skateboard motion after the event is very similar to that before. As the student steps onto the skateboard, student and skateboard experience equal and opposite forces. The same happens as the student steps off. The skateboard experiences first a forwards force and then a backwards force, and these are approximately the same.
4 In step d, there is little friction between the lower board and the skateboard. So the lower board is unable to exert much force on the skateboard. The student remains stationary relative to the ground.
This can also be shown with a 'normal' experiment trolley on a sheet of card placed on rollers. Pull the card suddenly. The trolley gains little motion and just drops off the card as it is moved.
5 In step e, forces are, as always, equal and opposite. Whichever student pulls, the other experiences a force of the same size and in the opposite direction, and the collision occurs at the same place.
This experiment was safety-checked in April 2006