Answer 2_1_5
A brick falls off the top of a wall under construction and drops into a bed of sand 14.5 m below. It makes a dent in the sand 185 mm deep. What is:
a) The speed of the brick just before it hits the sand.
b) Its deceleration in the sand.
c) What would happen to a person undergoing that deceleration?
a) The speed of the brick just before it hits the sand.
Use v2 = u2 + 2as = 0 + 2 x 10 m/s2 x 14.5 m = 290 m2/s2
Therefore v = 17 m/s
b) Deceleration in the sand:
Use v2 = u2 + 2as. 0 = 290 m2/s2 + 2 x a x 0.185 m
Rearranging:
2 x a x 0.185 m = - 290 m2/s2
a = 290 m2/s2 = 784 m/s2
2 x 0.185
c) What would happen to a person undergoing that deceleration?
784 m/s2 is about 78 g, quite sufficient to cause fatal injury.