1. Jan 23, 2010

### Shaley

sue drives her 1452 kg car along a straight road at a constant velocity of 30 m/s [E]. Ger brakes suddently give out. she puts the car in neutral and lets it coast for 25 seconds. The air drag decelerates the car to a velocityof 25 m/s [E]. assume a frictionless surface.

PART C IS WHERE IM CONFUSED, BUT YOU'LL NEED TO KNOW MY ANSWERS TO THE OTHER ONES FIRST.

b)determine average acceleration while car's decelerating

m=1452 kh
v1=30 m/s [E]
v2=25 m/s [E]
t=25s

d=1/2(v1+v2)(t)
=1/2(55)(25)
=688 m

a= v2squared-v1squared/2(d)
=25squared-30squared/2(688)
= -0.20 m/ssquared

b)determine average force of air against car

F=(m)(a)
=(1452)(-.2)
=-290.4N

c)after coasting for 25s, she pulls her e-brake to slow to the car to a stop. If it takes 3s to stop the car, what is the force applied by the e-brake? Assume that the force exerted by the air remains constant and is equal to the forst determined in part (b).

2. Jan 23, 2010

### kuruman

In part c there are two forces providing negative acceleration. You already know one from part b. To do part c first find the total acceleration. Call it a. Then say that the known force plus the force you are looking for are mass times the acceleration that you just found.