Basic constant acceleration help

1. Jan 26, 2009

CustardPi

Ok, I've been running over this problem for about an hour now, and I can't figure out where I'm going wrong. I'm a complete newb, so I'm sure it's something stupid, but I could use the help.

1. The problem statement, all variables and given/known data
An object with an initial velocity undergoes constant acceleration. Position information was collected at 4 poins and is shown in the table. Using Yn+1 - Yn = Vnt + (1/2)ayt2 determine the acceleration and fill in each box.

edit : it screwed up my table, I hope it makes sense.

Point #; Time (s); Position (m); Average Acceleration (m/s2
1; 0; 1; n/a
4; 3; 52; n/a

2. Relevant equations

Yn+1 - Yn = Vnt + (1/2)ayt2

3. The attempt at a solution

Here's one attempt I tried.

25m - 8m = 7(1) + 1/2ay(1)

ay = 20

I've tried a few different attempts, I'm not sure I should show them all here. The problem I'm running into is that I think I need to plug in velocity figures, but I wasn't given any, so I messed around some, but the answers don't make sense, and aren't uniform.

2. Jan 26, 2009

rl.bhat

At position 1 Yn = 1m, and Vn is the initial velocity.
At position 2 Yn+1 = 8 m and t = 1s
At position 3 Yn+1 = 25 m and t = 2s
At position 4 Yn+1 = 52 m and t = 3s
From position 1 and 2 you get 8-1 = V0 +1/2*a.........(1) Similarly wright the equations for positions 1 and 3, and 1 and 4. Solve the equations to find the acceleration.

Last edited: Jan 26, 2009
3. Jan 26, 2009

CustardPi

Ok, that all makes sense, but I'm still not sure what to do for initial velocity, since it wasn't given to us. Am I supposed to use some equation to find it?

4. Jan 26, 2009

rl.bhat

Wright three equators. From the first equation we get Vo = 7-1/2*a. Use this value in other two equations to find a

5. Jan 26, 2009

CustardPi

Thank you for your help, I figured it out earlier today, things are making more sense every day!