Find speed and acceleration of truck

1. Sep 21, 2007

012435

1. The problem statement, all variables and given/known data
a truck covers 40m in 8.50s while slowing down to a final speed of 2.80m/s

2. Relevant equations

find original speed
find it's acceleration

3. The attempt at a solution

for original speed i think it is 40m/8.50s= 4.7m/s
now for acceleration i am not sure which kinematic equation to use. i think it's Vf=Vi+at am i on the right track or no

2. Sep 21, 2007

learningphysics

That's not right... for original speed.

d = vt only applies for constant speed situations...

you need a different equation to find the original speed.

3. Sep 22, 2007

012435

so is the original speed then the same as the initial speed

4. Sep 22, 2007

bob1182006

yes, what equation do you know that relates velocity, initial velocity, and time?

5. Sep 22, 2007

012435

well for acceleration you would use a= (Vf-Vi)/t
for initial speed would you use a=2(x-V1t)/t^2

6. Sep 22, 2007

learningphysics

Yes, but the acceleration formula here requires initial speed... the initial speed formula requires acceleration... so neither of these formulas let you get started...

you can calculate original speed immediately. which formula lets you get it?

7. Sep 24, 2007

012435

can u set these two equations equal to each other, cancel acceleration and solve for Vi or am I making it more complicated

8. Sep 24, 2007

learningphysics

d = (v1+v2)/2 * t

You can use this to get the original speed.

9. Oct 1, 2007

012435

i get it ..thank u