Determining altitude given velocities and mass

[mandy9008]

Homework Statement

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.

Homework Equations

1/2mv² = mgy + 1/2mv²

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

 

[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.

 

[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

 

[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.

 

[mandy9008]

how did you get 4.935m?

 

[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.

 

[mandy9008]

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

 

[joshmdmd]

hmm I can't get 4.95..

i'm getting 5.01..

but your initial eq'n is right.

 

[mandy9008]

josh, i kept getting 5.01 as well