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stone is dropped from a cliff of height h at the same moment as another stone is thrown vertically upward from the bottom of the cliff with an initial velocity u. The stones are at the same horizontal level after a time t. Show the condition necessary for the stones to have equal speeds at this level.

Need hint.

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  • $\begingroup$ Have you tried simply writing down expressions for the heights of the two stones as a function of time, setting their heights equal to each other, and then seeing what their velocities are at that instant in time? $\endgroup$
    – user93237
    Commented Mar 30, 2018 at 19:36
  • $\begingroup$ I did expression for the heights as a function of time. But stumped at their velocities. $\endgroup$
    – Boboo
    Commented Mar 30, 2018 at 21:35

1 Answer 1

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You could try thinking in terms of kinetic and potential energy. If they have the same speed at a given level that means that their potential energies are equal and their kinetic energies are equal too. Think of initial conditions that would satisfy this at some point.

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    $\begingroup$ From the way the problem is stated, it doesn't appear that the two stones are required to have the same mass so they don't necessarily have the same potential energy at the instant that they have the same height. Same thing with their kinetic energies. $\endgroup$
    – user93237
    Commented Mar 31, 2018 at 0:57

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