# Homework Help: Determining altitude given velocities and mass

1. Jun 22, 2010

### mandy9008

1. The problem statement, all variables and given/known data
A 52 kg pole vaulter running at 10 m/s vaults over the bar. Her speed when she is above the bar is 1.3 m/s. Neglect air resistance, as well as any energy absorbed by the pole, and determine her altitude as she crosses the bar.

2. Relevant equations
1/2mv2 = mgy + 1/2mv2

3. The attempt at a solution
1/2 (52kg)(10 m/s)2 = 52kg (9.8 m/s2) y + 1/2 (52kg)(1.3 m/s2)
y=51.8m

2. Jun 22, 2010

### rl.bhat

1.3 m/s is the horizontal component of her velocity. Initial velocity is given. Find the vertical component of the velocity and then find the height.

3. Jun 22, 2010

### mandy9008

1.3^2 + b^2 = 10^2
b=9.91

the equation i used was:
1/2 (52kg)(10 m/s)2 = 52kg (9.8 m/s2) y + 1/2 (52kg)(9.91 m/s2)
y=0.091m

4. Jun 22, 2010

### wwshr87

Your initial equation is correct and it will give you 4.935m as the height for which she jumps. When looking into conservation of energy you do not deal with vectors only magnitudes.

5. Jun 22, 2010

### mandy9008

how did you get 4.935m?

6. Jun 22, 2010

### wwshr87

I just solved the equation you had in the beginning. I am pretty sure the method is correct, check your math.
1/2 (52kg)(10 m/s)2 = 52kg (9.8 m/s2) y + 1/2 (52kg)(1.3 m/s)^2, solve for y.

7. Jun 22, 2010

### mandy9008

hmm, i must have put something in wrong. 4.935 was correct. Thanks

8. Jun 22, 2010

### joshmdmd

hmm I can't get 4.95..

i'm getting 5.01..

but your initial eq'n is right.

9. Jun 22, 2010

### mandy9008

josh, i kept getting 5.01 as well