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This is from "3000 solved problems in physics". Yes, I have the solution but

I still have a question.

**a ball is thrown vertically upward with a velocity of 20 m/s from a top of a tower**

having height of 50m. On its return it misses the tower and finally hits the ground.

having height of 50m. On its return it misses the tower and finally hits the ground.

**What time t elapses from the moment the ball is thrown until it passes the edge of the tower?**

The book solution is clear, it chooses y=0 to be at the top of the tower.

Then: y=v

_{o}t + (1/2)at

^{2}

This is solved to give t=0 (the beginning) or t=4.08s.

OK, nice, but I'd like the y axis to be at the ground. Clearly, I cannot apply the above equation,

because it will neglect the additional potential energy that the ball have when now it's 50m above

the y axis. (If I substituted 50 for y in the eqn, I'll get a non-solvable eqn, or, if you like, an imaginary solution).

So how can we solve this (without using the concept of energy) ?