# velocity/acceleration

• Aug 9th 2009, 09:39 AM
skeske1234
velocity/acceleration
A dragster races down a 400 m strip in 8 seconds. Its distance, in metres, from the starting line after t seconds is s(t)=6t^2+2t

a) find the dragster's velocity and acceleration as it cross the finish line.

b) how fast was it moving 60 m down the strip?

I need help with part b.. I thought that i would use the acceleration formula i made up in part a but it did not work out since the a(t) formula in part a is a constant number > 12

Help with part b. thanks
• Aug 9th 2009, 09:42 AM
Jhevon
Quote:

Originally Posted by skeske1234
A dragster races down a 400 m strip in 8 seconds. Its distance, in metres, from the starting line after t seconds is s(t)=6t^2+2t

a) find the dragster's velocity and acceleration as it cross the finish line.

b) how fast was it moving 60 m down the strip?

I need help with part b.. I thought that i would use the acceleration formula i made up in part a but it did not work out since the a(t) formula in part a is a constant number > 12

Help with part b. thanks

how did you do (a)?

For (b), set s(t) = 60 and solve for t. Then plug that t-value into the velocity equation to find its velocity at that time (why would you use acceleration??)
• Aug 9th 2009, 09:49 AM
All you need to do is set $s(t)=6t^2+2t=60m$ and use the quadratic formula to solve for $t$. Take the solution and plug it into $s'(t)=12t$ to obtain the velocity at 60m. If you don't remember the formula to solve $6t^2+2t-60=0$ for $t$, just look it up on wikipedia. Or just factor the equation. I'm bad at factoring, so I always just use the quadratic formula.