# Stop Sign Question

1. Jan 16, 2012

### billybobay

1. The problem statement, all variables and given/known data
When Bob applies the brakes to his car, it slows uniformly from 20.0 m/s to 0.0 m/s in 4.0 s. How many meters before a stop sign must he apply his brakes in order to stop at the sign?

80 m

60 m

40 m

20 m

2. Relevant equations

d=(20+0)/2(4)
d= 40 m

3. The attempt at a solution

Bob needs 40 m to stop his car.

Would that be correct?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 16, 2012

### SammyS

Staff Emeritus
Yes.

3. Jan 16, 2012

### PotentialE

Distance = ??
Time = 4s
Velocity initial = 20m/s
Velocity final = 0m/s
Acceleration = ??

A = Vf-Vi / T
A = -20 / 4
A = -5m/ss

D = ViT + ATT/2
D = (20)(4) + (-5)(4)(4) / 2
D = 80 + -40
D = 40m

yes, you're correct

4. Jan 16, 2012

### SammyS

Staff Emeritus
@ billybobay,

It's perfectly fine to work it out the way you did, using average velocity.