If mass m1 has speed v, then it has kinetic energy m1v^2. Mass m2 has 0 speed so initial kinetic energy 0. Taking potential energy to be at be bottom of the pendulum, where the two masses first strike, the total energy of the system is m1v^2. At height h= 20 cm, the system potential energy is h(m1+ m2)g and, since two masses stop there, the system kinetic energy is 0. Since the total energy is conserved, we must have m1v^2= h(m1+ m2)g. Solve that equation for v.
(Because the two objects stick together their collision is NOT "elastic" and kinetic energy is not conserved but the total energy of the system is conserved.)
It is hard to tell what "m1" and "m2" are because, in your textbook, the two masses are called "m1" and "m2" in the picture but "m" and "M" in the text.
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Last edited by HallsofIvy; Apr 10th 2018 at 02:58 PM.
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