'Testing rocks - 1 bouncing back testing 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 we 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.
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