Last question and I dont understand it :( physics help?

Question

A driver a car traveling at 15.0 m/s applies the brakes, causing a uniform acceleration of -2.0 m/s^2. how long does it take the car to accelerate to a final speed of 10.0 m/s? How far has the car moved during the braking period?

Answer 1

Favorite Answer

use eqn

vf=vi+at

t=(vf-vi)/a

t=(10-15)/(-2)

t=2.5 s

since you know time you can use eqns

d=0.5(vi+vf)t

d=vit+0.5at^2

but ill just use an eqn that doesnt use time

vf^2=vi^2+2ad

d=(vf^2-vi^2)/ 2a

d= [(10^2)-(15^2)]/ 2(-2)

d=31.25

Answer 2

vf^2-vo^2=2ax

10^2-15^2=2(-2)x

100-225=-4x

-125=-4x

31.25 m=x

vf=vo+at

10=15+-2)t

-5=-2t

2.5 s=t