# Speed and acceleration

1. Feb 26, 2008

### chocolatelover

1. The problem statement, all variables and given/known data
A truck covers 45.0 m in 8.50 s while smoothly slowing down to a final speed of 2.20m/s
a. Find its original speed
b. find its acceleration

2. Relevant equations

average velocity=xf-xi/change in t
average acceleration=vf-vi/chang in t
final velocity=vi+at

3. The attempt at a solution

____________________________

0 8.5 s
2.20m/s

x-2.20m/s/(8.5-0s)

I know that the entire distance is 45.0 m. I don't think I can use any of the three formulas mentioned above, but I'm not sure which other one I could use. Could someone please show me what formula I need or give me a hint, please?

Thank you very much

2. Feb 26, 2008

### e(ho0n3

Since the truck is "smoothly slowing down", the acceleration must be constant and negative.

There are many nice formulas involving distance and speed with constant acceleration. Which ones do you know?

3. Feb 26, 2008

### XxBollWeevilx

You're on the right track with some of the formulas you wrote. Think kinematics.

4. Feb 26, 2008

### chocolatelover

Thank you very much

I know that vf=vi+at, so vi must be=vf-at

acceleration=vf-vi/change in time

Therefore, vi=vf-[(vf-vi)/(change in time)*(t)] right? This doesn't seem to help.

I know that vf=2.20m/s and the change in time is 8.5. It seems that I need to know what vi or a is. 2.20m/s=vi+a(8.5) or would it just be v=d/change in time? vi=45/8.5=5.3
vf=vi+at
2.2=5.3+a(8.5)
a=-3.65

Could someone please explain to me how I could find vi, if I don't know what the acceleration is, please?

Thank you very much

Last edited: Feb 26, 2008
5. Feb 26, 2008

### cse63146

You would just use this formula

d=$$\frac{V_i+V_f}{2}$$*t

Last edited: Feb 26, 2008
6. Feb 26, 2008

### chocolatelover

Thank you very much

Does this look right?

d=vi+vft/2

45=vi+2.2m(8.50)/2

vi=71.3

a=vf-vi/t

a=2.2-71.3/8.5
=-8.13

Thank you

7. Feb 26, 2008

### cse63146

sorry didnt realise how the equation came out. You multiply time t by (V_f+V_i)/2

8. Feb 26, 2008

### chocolatelover

It's okay. Does this look right now?

45=8.5vi+18.7/2

vi=8.34

a=2.2-8.34/8.5=-.72

Thank you

9. Feb 26, 2008

### cse63146

Looks good to me, but I would get another person to check it just in case because I could be wrong.

10. Feb 26, 2008

### chocolatelover

Thank you very much

Regards