Constant acceleration problem

1. Jul 25, 2016

Erenjaeger

1. The problem statement, all variables and given/known data
A motorcycle has a constant acceleration of 2.80 m/s2. Both the velocity and acceleration of the motorcycle point in the same direction. How much time is required for the motorcycle to change its speed from (a)40.9 to 50.9 m/s, and (b)70.9 to 80.9 m/s?

2. Relevant equations
Vf = Vi + a*t

3. The attempt at a solution
using that kinematic formula,
for a) 50.9=40.9+2.8*t
then divide both sides by 40.9+2.8 to solve for t

and same thing with b) but in both cases the answer is coming up as wrong. what have i done wrong?

2. Jul 25, 2016

collinsmark

$50.9 = 40.9 + 2.8t$

is not the same thing as

$50.9 = (40.9 + 2.8)t.$

And for that matter, neither one gives an answer of 4.04 sec. So I'm not really sure what happened there.

(Hint, you can't just add 40.9 plus 2.8 that way. That's not the way algebra works.)

3. Jul 25, 2016

Erenjaeger

oh right, so basically i have to get 2.8t on its own and then divide by 2.8??

4. Jul 25, 2016

collinsmark

Yes, that is the correct way.

5. Jul 25, 2016

Erenjaeger

cool thanks!!