2005 1(b)        A mass of 8 kg falls freely from rest. After 5 s the mass penetrates sand.
The sand offers a constant resistance and brings the mass to rest in 0.01 s.
Find

(i) the constant resistance of the sand

(ii) the distance the mass penetrates into the sand.

ANSWER

First find the speed at which the mass strikes the sand after falling freely from rest.
(Take down to be positive.)

    No “s”:  

Now to find the deceleration in the sand.

    No “s”    i.e.   i.e. a = -4900 ms^-2

The forces acting on the mass are its weight and the resistance of the sand.

Sum of forces = ma

   i.e.      i.e. R = 39 278.4 N

(ii)