A person of mass 60kg is standing on a raft of mass 40kg in a lake. The distance of the person from the bank is 30m. If the person starts running towards the bank with velocity 10m/s, then what will his distance be from the bank after one second?

When the man starts running with velocity 10 m/s towards the bank, the raft will move away from the bank at velocity, say, u. Such that,

M(10) + mu = 0 [by law of conservation of momentum]

=> (60)(10) + (40)u = 0

=> u = -15 m/s [opposite to the direction of man]

So, the velocity of the man towards the bank w.r.t. the ground will be = 10 – 15 = -5 m/s [away from the bank]

In 1 s he travels 5 m away from the bank. So, after 1 s, he is 30 + 5 = 35 m from the bank.