I don't want you to give me the answer of the problem, I only whant to know how to think it.

A frog of 200g is sitting at the end of a plank of 1kg and 1 m of length. The plank is at rest and floating on the calm waters of a pond. Suddenly the frog jump along the board. If the frog falls at the other end of the plank, calculate the horizontal distance traveled by the frog. The friction between the board and the water is neglected and the frog can be considered as a point mass.

Because in fact I have an idea on how to solve it ,but I think the question is giving me the aswer in some way and I find it quite strange..beacuase it's an exam question ,and if that's the case it would be really easy.

  • 1
    $\begingroup$ What is your solution, which should be given from the question text? $\endgroup$
    – Steeven
    Commented Nov 10, 2017 at 18:59
  • $\begingroup$ @Steeven, I guess the author thinks that the length of plank is the answer. $\endgroup$
    – Tajimura
    Commented Nov 11, 2017 at 19:18
  • $\begingroup$ Yes ,I don't understand what the problem is asking... $\endgroup$
    – Emmaaaaa
    Commented Nov 11, 2017 at 22:00
  • $\begingroup$ I know I'm wrong ,thats why I am asking this $\endgroup$
    – Emmaaaaa
    Commented Nov 11, 2017 at 22:01
  • $\begingroup$ But ,what shufflle answer made me figure it out. $\endgroup$
    – Emmaaaaa
    Commented Nov 11, 2017 at 22:03

1 Answer 1


Consider that because of conservation of momentum, since the entire system is at rest initially, the entire system will be at rest at the end when the frog lands. And therefore the horizontal center of mass of the system will have not changed, but the system will appear mirrored from its initial setup.

So, if you can figure out where the horizontal center of mass for the system is, your answer should follow easily.


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