# Acceleration due to Gravity: calculate initial velocity

1. Oct 21, 2008

### Archedcutie

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

A Pole vaulter leaves the ground to travel 4.0 m up and over the bar, and then another 4.0 m back to the ground. It takes 1.81 s for the whole trip. Caculate the velocity with which she left the ground.

2. Relevant equations

Well the equation I used which I saw in my textbook was, v1[initial speed] = d-1/2[g][t]^2

3. The attempt at a solution

I subbed it in as v1= 8m- 1/2 [9.8N/kg][1.81s]^2
and I didn't get 8.8m/s [up] like the answer at the back of the textbook :(

2. Oct 21, 2008

### aimslin22

I think 8m in the equation is wrong. How I read the problem is she travel up (y) 4 m, and then down 4 m, so the d = 4m. maybe that is it? I am not too sure, but maybe it's worth a try.

3. Oct 21, 2008

### einstein_a_go_go

Hi,

if you forget 'THE equation' approach and think back to first principles.

The whole journey takes 1.81 s.

How long will the up part of the journey take?

The vaulter starts at a speed v, vertically.

At the peak of the vault what is their vertical speed?

now, you should be able to work it out as you have end speed, acceleration, distance and time!

Cheers

4. Oct 21, 2008

### einstein_a_go_go

on the aimslin22 front:

they travel up 4m and down 4m - so their overall displacement (Not distance) is....