# Homework Help: Fairly simple (I assume) acceleration derivation I've blanked on

1. Nov 28, 2011

### Jonnoley

Hi, I understand this is probably not at all complicated, but I've completely blanked and I've been working at it for hours now.

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

Derive the expression g=2(L2t1-L1t2)/t1t2(t1+t2)

I've replaced the lower case ls there with upper case one's for clarity, but it's just two distances, L1 and L2

This relates to a simple experiment to measure g I'm currently doing a lab report for, where L1 and L2 are the distances between some infrared sensors on the drop tower, and t1 t2 are the times for the ball I dropped to pass between the sensors.

2. Relevant equations

I have no idea, I've tried every equation of motion and every mathematical operation I could.

3. The attempt at a solution

I have tried substituting Δt, ΔL and Δv into L=vt+(at2)/2 and have got some fairly close answers, but have also proved acceleration doesn't exist several times, so that might be barking up the wrong tree, in the wrong forest, on the wrong planet.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 28, 2011

### Steely Dan

This should be in the lower level physics forum, but no matter. Another relevant equation that you might want to try using is $$v_f^2 = v_0^2 - 2 g d.$$ That should help you eliminate some of the velocities (that you don't ultimately care about) from the other kinematics equation you listed.

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