# Ball in free-fall

1. Apr 3, 2009

### aaronb

1. The problem statement, all variables and given/known data

A ball of mass m = 8 kg is dropped from rest at a height h = 13.9 m above the ground. Ignore air resistance.

(a)If the ball is being released with a downward speed 4.9 m/s initially, what will be its final speed when it hits the table 0.7 m below the release point?

(b) If the ball is being thrown upwards instead with the same speed 4.9 m/s, what is the final speed when it hits the table?
2. Relevant equations

Kf + Ugf = Ki + Ugi

3. The attempt at a solution
vf = sqrt(v(initial)^2 + 2g(h - y))
I just plugged in numbers and got 16.8 for part (a) and I plugged in numbers for part (b) and got 17.3. Also, for part (b), I substituted y=0 because I chose the y-axis to start at the bottom where the hand would be.

2. Apr 3, 2009

### LowlyPion

To be clear. The ball is released at 13.9m and it hits the table at .7m?

And the initial velocity is 4.9m/s.

What does dropped from rest mean when you have an initial velocity given?

3. Apr 3, 2009

### rl.bhat

vf = sqrt(v(initial)^2 + 2g(h - y))
In the first part the distance traveled by the ball is only 0.7m.

4. Apr 3, 2009

### aaronb

Ignore that released from rest part.