Question

A man weighing $80kg$ is standing at the centre of a flat boat and he is $20m$ from the shore. He walks $8m$ on the boat towards the shore and then halts. The boat weight $200kg$. How far is he from the shore at the end of this time?

• A. $11.2m$
• B. $13.8m$
• C. $14.3m$
• D. $15.4m$

Answer 1

The correct option is C $14.3m$

As you can see there is no net displacement of centre of mass of the whole system as the boy moves toward the shore boat moves to the opposite side to balance the centre of mass$.$

Say that the boat moves by distance x away from the shore therefore boy moves $x-8$ as you can see there is no net external force delta x of centre of mass is zero$.$

therefore$,M_1{x}_1+M_2{x}_2=0$

$200\left(x\right)+80(x-8)=0$

$280x=640x=2.3$

Therefore boy moves $8-2.3=5.7$ towards the shore

Now the total distance between the shore and the boy is $20-5.7=14.3m$

Hence,

option $\left(C\right)$ is correct answer.