Constant Acceleration; answer not correct for some reason?

[dudenamedscott]

Homework Statement

A motorist suddenly notices a stalled car and slams on the brakes, negatively accelerating at 6.1m/s2 . Unfortunately this isn't enough, and a collision ensues. From the damage sustained, police estimate that the car was going 22km/h at the time of the collision. They also measure skid marks 35m long. Aow fast was the motorist going when the brakes were first applied? B: How much time elapsed from the initial braking to the collision?

Homework Equations

2 a (x - xo)= v2 - vo2
v(average)=d/t

The Attempt at a Solution

A: 2(6.1m/s^2)(35-0m)=(55/9m/s)^2-vo^2
vo^2=19.7m/s

B: 19.7m/s=35/t
t=1.78sec

I may just be totally off, but I feel like I understood the problem. Masteringphysics doesn't agree. Thanks again haha.

 

[lightgrav]

you mean v = 19.7 m/s ? ... are acceleration and displacement in the same direction?
what happened to the minus sign in front of vo^2 ?

 

[PhanthomJay]

Homework Statement

A motorist suddenly notices a stalled car and slams on the brakes, negatively accelerating at 6.1m/s2 . Unfortunately this isn't enough, and a collision ensues. From the damage sustained, police estimate that the car was going 22km/h at the time of the collision. They also measure skid marks 35m long. Aow fast was the motorist going when the brakes were first applied? B: How much time elapsed from the initial braking to the collision?

Homework Equations

2 a (x - xo)= v2 - vo2
v(average)=d/t

The Attempt at a Solution

A: 2(6.1m/s^2)(35-0m)=(55/9m/s)^2-vo^2
vo^2=19.7m/s

B: 19.7m/s=35/t
t=1.78sec

I may just be totally off, but I feel like I understood the problem. Masteringphysics doesn't agree. Thanks again haha.

If you take the displacement as positive, say to the right, in which direction is the acceleration? (Check your signage). Then once you get the correct value for the initial speed, you need to calculate the average speed to find the time in part b.