Monday, 11 February 2013

Testing the strength of rocks - bouncing back

The new ELI is 'Testing rocks - 1 bouncing back; investigating the strength of rocks'. When engineers build structures such as dams, roads and tunnels, they need to investigate the properties of the rocks beneath and around them. One of the key properties is the strength of the rocks. This normally requires expensive equipment, but in this activity pupils can get quite a good idea by simply dropping a ball bearing onto a flat, cut sample of the rock. The height to which the ball bearing bounces back allows us to compare the relative strength of different rocks.
This is one many innovative teaching activities which you can download free from our website.


Unknown said...

Not strength. At least not until the impact is high-enough energy to actually deform plastically or fracture grains or grain interfaces.
Elasticity, of the several modes of elastic deformation, is the 'bounce' in the impactor.

Earth Learning Idea said...

Explanations have been written in the 'Underlying principles' within the activity, as follows:-
There is a statistical correlation between bounce height and rock strength. “Rock strength”, as used here, is a shortened version of “the uniaxial compressive strength of a rock”, measured in megapascals. Engineers would use equipment such as a Schmidt Hammer, (see photograph), which uses a rebound measurement to obtain the uniaxial compressive strength.
An elastic material has the ability to recover strain without loss of energy, i.e. the stress/strain curve is followed downwards when stress is reduced. Again, there is a good statistical relationship between elasticity as defined above and material strength.