Calculate Distance for Bob to Stop at Stop Sign

[billybobay]

Homework Statement

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

Homework Equations

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

The Attempt at a Solution

Bob needs 40 m to stop his car.

Would that be correct?

 

[SammyS]

Yes.

 

[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

 

[SammyS]

@ billybobay,

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