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I found a set of question from Harvard here;

https://www.physics.harvard.edu/academics/undergrad/problems

I solved the Week 1 problem like this;

The basketball would fall to the floor, because the collision is elastic the velocity will change from -v to v (where v is \\sqrt{2gh}\. There for the change of momentum is \(2Mv\) (where M is the mass of the basketball).

The tennis ball therefore would have the sum of the change of momentum which would be

\2Mv+2mv\, and tennis ball's velocity would satisfy the equation

\2Mv+2mv=mv+m{v}'\ (when the tennis ball's velocity after collision is \-{v}'\)

According to this \{v}'\ should be \\frac{(2M+m)\sqrt{2gh}}{m}\

and further calculation tells me that h equals \(\frac{2M+m}{m})^{2}h\

If you take a look at the solution (on the right hand side) you would find out that this is wrong, what did I do wrong in solving this problem? Why can't the change of momentum be used?

https://www.physics.harvard.edu/academics/undergrad/problems

**[URL repaired by a mentor]**I solved the Week 1 problem like this;

The basketball would fall to the floor, because the collision is elastic the velocity will change from -v to v (where v is \\sqrt{2gh}\. There for the change of momentum is \(2Mv\) (where M is the mass of the basketball).

The tennis ball therefore would have the sum of the change of momentum which would be

\2Mv+2mv\, and tennis ball's velocity would satisfy the equation

\2Mv+2mv=mv+m{v}'\ (when the tennis ball's velocity after collision is \-{v}'\)

According to this \{v}'\ should be \\frac{(2M+m)\sqrt{2gh}}{m}\

and further calculation tells me that h equals \(\frac{2M+m}{m})^{2}h\

If you take a look at the solution (on the right hand side) you would find out that this is wrong, what did I do wrong in solving this problem? Why can't the change of momentum be used?

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