Question

A block of metal wheighing 2 kg is resting on a frictionless surface,if struck by a jet releasing water at the rate of 1kg/s and at a speed of 5m/s.what is the initial acceleration of the block ?

Answer 1

The final velocity of mass of water after hitting the metal block is zero.

So the average change in momentum per sec = initial momentum
`="mass"xx"initial velocity of water"`

`=1kgxx5m"/s"=5kgm"/s"`

As this change in momentum occurs during 1 sec. We may say that the force exerted on the 2kg metal block is
`F="change in momentum"/"time"`
`=(5kgm"/s")/(1s)=5N`

Hence by Newton's law initial acceleration of the block of mass `m=2kg` is given by

`a=F/m=(5N)/(2kg)=2.5m"/"s^2`