Question

Julie is traveling in a car at 72.0 km/h when the car is involved in an accident that brings Julie and the car to a stop. Assume that Julie's mass is 40.0 kg.

A) If Julie is wearing her seat belt, she comes to rest in a time of 0.20 seconds. What is the magnitude of the average force (in Newtons) exerted on Julie (mostly by the seat belt) as she is slowing down?

B) What is the magnitude of Julie's average acceleration as she is slowing down? Express this in units of g, assuming g = 10.0 m/s/s.

C) If Julie is not wearing a seat belt, and the car does not have an air bag on the passenger's side, where Julie is, she may come to rest in a time of 0.020 s, after striking the windshield, instead. What is the magnitude of Julie's average acceleration in this case? Again, express this in units of g.

Answer 1

A) we know the formula

F=P/t..............1)

we have , m=40 kg and v=72km/h= 72*5/18=20m/s

p=mv=40kg*20m/s=800Ns

t=0.20s

so now using eqn 1 we have

F=800Ns/0.20s=4000N

so the answer is 4000N

B)we know the formula

F=ma

a=F/m=4000N/40kg=100m/s²=100/10=10 g

so the answer is 10 g

C) the new speed of julie now is

V=vi-at

here time t=0.12s,

first we have to take the velocity   of julie in 0.1 seconds and then slowing down to 0.02 s, so total time is 0.12 s

v=20m/s-100m/s²*0.12s=8m/s

now again using eqn 1

F=P/t=*40 kg*8m/s)/0.02=16000N

and a=F/m=16000N/40kg=400m/s²=400/10=40 g

so the answer is 40 g