# Finding velocity after collision if you know mass and height. Help

Finding velocity after collision if you know mass and height. Help!!

## Homework Statement

A 50 gram ball enters a a pendulum with mass 200 g. The pair then swings up to a height of 11 cm.
find the velocity immediately after the collision? Then find initial velocity of ball before collision.

## Homework Equations

Vf= square root of 2gh.

## The Attempt at a Solution

Used above equation and got 14.7 which proved incorrect. Any help would be nice! Thanks!!

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Doc Al
Mentor
Used above equation and got 14.7 which proved incorrect.
You're off by a factor of 10, so I suspect a problem with units. Try again!

Ahh forgot to change 11 cm to meters. My answer of 1.47 proved to be correct! Thanks so much

So I have 1.47 m/s for my velocity after the collision. How do I find initial velocity of just the ball right before the collision?!

I tried the equation Vf^2= Vi^2 + 2gh but my answer proved incorrect. The answer I calculated was .07 m/s

Doc Al
Mentor
So I have 1.47 m/s for my velocity after the collision. How do I find initial velocity of just the ball right before the collision?!
What's conserved during the collision?

Momentum? Law of conservation of mometum?... I don't have a clue!

Doc Al
Mentor
Momentum? Law of conservation of mometum?
Exactly.

... I don't have a clue!
Sure you do. I used the equation Vf= M1 x Vi / M1 + M2
1.47 = .05 kg x Vi / .05 + .2 = 7.35.
My answer was correct!! Thanks so much :)