Vertical Jump Analysis: Calculating Time and Take-Off Speed

[KaseyK]

Homework Statement

A certain volleyball player can jump to a vertical height of 85 cm while spiking the volleyball.

Homework Equations

(a) How long is the player in the air for?
(b) What is the player's vertical take-off speed?

The Attempt at a Solution

Firstly I converted 85 cm to m, which is 0.85m.

I then drew a free-body diagram to help me visualize what was happening with the player. Sorry I can't draw it on here.

I then used the kinematic equation:

d=v1(t) + 1/2(a)(t^2)
Because I assumed that the inital velocity was zero (because he hasn't jumped up yet) the equation was left with:

d=1/2(a)(t^2)
.85x2 = 1/2(9.81)t^2
t = 0.61 seconds

The answer should be 0.83 seconds. Where did I go wrong?

I haven't attempted 'b' because I need to figure 'a' out first.

Thanks in advance for your help.
Kasey

 

[blackboy]

Initial velocity is not 0 because if she started on the ground, how did she get in the air? You have two variables, v0 and t.

 

[KaseyK]

Okay did I at least use the right kinematic equation?

Is there anything else you can hint to, to send me in the right direction?

Thanks

 

[blackboy]

There are multiple ways of doing this. Yes your way is valid. Remember the time it takes to come down is the same as the time it takes to come up.

 

[Cyosis]

You can't calculate it like that because you don't know his initial speed yet. What you do know is his speed when he reaches 85cm height. What is it? How long does it take him to fall from 85cm back to the ground?

Edit: I made an assumption here that you have to solve it using kinematics. If this is not so then there is an easier way!

 

[cepheid]

Because you have two unknowns, you need two equations. For the second equation, remember that the velocity has been reduced to zero at the top of the jump.

 

[KaseyK]

You can't calculate it like that because you don't know his initial speed yet. What you do know is his speed when he reaches 85cm height. What is it? How long does it take him to fall from 85cm back to the ground?

Edit: I made an assumption here that you have to solve it using kinematics. If this is not so then there is an easier way!

No I don't need to solve it using kinematics. I can use any way I want, I just can't figure either way out...ugh.

 

[Cyosis]

We can do it multiple ways, but let us first do it in the order you're asked to do it.

You need to answer this. What is the speed of the player at 85cm and how long does it take for him to fall 85cm after reaching that height?

 

[blackboy]

Sorry when I said your way is valid, I was talking about your equation. Sorry for any confusion that it caused.