Q:

I work in a hardware store and am doing self-study of Physics from an old textbook. I am NOT a student of physics in any college/school yet. I am stuck with one problem, and although I can somehow reach the right answer, it does not match my foundational grasp.
This a Conservation of Momentum" problem, gathered from an old textbook. I thought it would be simple, but I seem to be goofing up somewhere in my basic conceptual understanding. Your help will be gratefully appreciated. Here it is:
PROBLEM
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" A 1.0 kg steel ball is dropped from 4.0 m above the floor. It strikes the floor and rises to max height of 2.5 m. Find the momentum transferred TO the floor FROM the ball."
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My issues: If I tried to solve it using Y axis as a directional path and take vector sign into account, I end up with the wrong answer. However, if I just take into account the ball's "p" immediately before and after the collision I CAN get the right answer. But I cannot reconcile that with my conceptual understanding. Please help.

- Tariq (age 37)

Berkeley, CA, USA

- Tariq (age 37)

Berkeley, CA, USA

A:

Hello Tariq,

You just have to repeat the mantra 'momentum is conserved' . That means if the ball loses momentum the floor has to take it up. In this case energy is not conserved but momentum is always conserved. The energy loss goes into a slight heating of the floor where the ball made what is called an inelastic collision. So you calculate the ball's momentum right before and right after the collision and add them up, keeping account of the vector nature of momentum. So your second method of calculating the momentum transfer is the correct one.

LeeH

*(published on 11/28/2014)*