# Predict value of g from displacement equation

1. Sep 10, 2008

### cb3930

1. I have to predict g, assuming that the correct equation for displacement as a function of time is x= 1/2gt^2. My equation for the displacement collected from experimental data is y = 0.30x^2 + 0.63x - 0.05. I am not sure how to find what g would be from these two equations or how they relate, but my guess would be try to isolate x from my equation perhaps? However, I need lots of help before Friday! Thanks in advance.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 10, 2008

### danago

What exactly does your experimentally obtained equation represent? What is y and x?

3. Sep 10, 2008

### Pythagorean

try to be a little more descriptive about what you're doing and be careful about your notation. (I think by x = 1/2gt^2, you mean x = (1/2)gt^2)

Also try to describe these quantities more physically (what does g, x, and y represent in physical reality? is this a lab?). Try asking the question again.

4. Sep 10, 2008

### cb3930

I apologize for the clarity issues. The lab was basically having a washer fall around 2m and little dots were made on a ticker tape that was threaded through the washer. X here is the displacement (m from the first point on the ticker tape). Y represents (t, which is the elapsed time in seconds). G in the equation x = (1/2)gt^2 is the constant acceleration due to gravity, but I am supposed to predict my value for g somehow and not use the set value (9.80 m/s/s)

Last edited: Sep 10, 2008
5. Sep 10, 2008

### LowlyPion

How were the dots made?

Last edited: Sep 10, 2008
6. Sep 10, 2008

### cb3930

The timer had a point that pressed down every 1/60th of a second on carbon paper which made dots on the ticker tape as it fell.

7. Sep 10, 2008

### LowlyPion

Any chance that would contribute to any measurement uncertainty - like slowing it up a wee bit?

8. Sep 10, 2008

### cb3930

Yes, in fact one the questions is to discuss the possible sources of error in my reported velocity values. I would suspect it might also influence what my predicted g value is. Does anyone have any idea how to get a g value from my equation?

9. Sep 10, 2008

### LowlyPion

Your data should show increasing speed.

Speed is the incremental distance - dot to dot/ time. If they are spaced at 1/60 of a second the distance dot to dot divided by 1/60 is your instantaneous speed.

Acceleration is the rate of change in speed.

So look at the differences between each of the successive dot to dot differences.

Example:
1 - 1.1 - 1.3 - 1.6 - 2.0 ... are where the dots measure to
Differences:
.1 - .2 - .3 - .4 ... These are velocities.
Differences of the differences:
.1 - .1 - .1 .....This is your acceleration.