Force of impact on a floor
This demonstration models a student jumping from a table to the floor. The force which the floor must exert to stop a ball of Plasticine quickly is measured.
Apparatus and materials
For step a:
Ball of Plasticine, 250 g
Domestic balance, 50 N
For step b:
Demonstration spring balance, 50 N
Metal sphere, 2 cm
Retort stands, 2
Extra retort stand rod
Ball of pPlasticine, 250 g
Steel rods, 2
Health & Safety and Technical notes
Set up a length of wood about 25 cm long, 7.5 cm wide and not more than 1 cm thick. Its weight will be neglected so it should not be heavy. Screw hooks fixed into one end pivot around a length of 5 mm steel rod held horizontally in a boss. This forms the pivot for the table.
Towards the other end of the beam use another screw hook to support the table from a demonstration spring balance fixed above it.
Position a second steel rod, R, held by a boss, so that the table is secured against it whilst the spring balance is under tension. Tie about 7.5 cm of twine to the metal sphere and trap about 5 cm of the end of the twine between the steel rod and the table. This sphere acts as a signal ball which will fall when the table moves downward and there is no longer a reaction between table and rod.
By adjusting the height of the demonstration spring balance, the tension can be altered within wide limits, a suitable value being 30 N.
a Drop a ball of Plasticine onto the domestic balance.
b Drop a ball of Plasticene onto the table from various heights until a height is found at which the signal ball is released. Take care to drop the Plasticine as near to the hook attached to the spring balance as possible and always on the same point on the table.
1 Step a gives a quick idea of the forces involved.
Step b shows that the force to release the platform is much greater than the weight of the Plasticine ball.
2 When the table is pushed down, releasing the sphere, there is no reaction between the rod R and the table and the force of impact P must exceed the tension T in the spring balance (neglecting the weight of the beam).
3 Students need practice in using the concept of momentum before they go on to conservation of momentum. Examples of using a small force for a long time, such as crumple zones in cars and bending knees when jumping from a great height, are helpful in discussion.