# physics pls help?

Consider a situation in which you are driving at 75 km/h when you notice a stalled vehicle 48 m directly ahead of you. Your brakes can apply a constant acceleration of 4.8 m/s2 [opposite the direction of motion]. (a) What is the distance you will travel while braking? (b) To avoid a collision, how much time do you have, after noticing the stalled vehicle, to hit the brakes

### 3 Answers

- Andrew SmithLv 75 months ago
I always laugh at these. a) How come you didn't see this vehicle until the last moment? b) how did you know it was exactly 48 m ahead? Did you leap out of your car and measure it? c) the legal minimum specification for brakes here is 6 m/s^2. Why were you driving an unroadworthy vehicle? Stay tuned for more exciting episodes of "How not to drive a car".

To answer the question convert the speed to m/s 75km / 1 hr -> 75000m/3600 s ~= 20.8 m/s

Time to stop = speed / acceleration = 20.8 / 4.8 = 4.34 s

Distance to stop v^ 2 = 2as -> s = v^2/(2*a) = 20.8^2/(2*4.8) =45.2m

You must reach the brakes before the vehicle has traveled 2.8 m

Time = distance / speed = 2.8 / 20.8 = 0.134 s which is faster than any human reaction so it is much too late you are already dead. Bye Bye.

- oubaasLv 75 months ago
braking time t = (0-75)/(3.6*-4.8) = 4.340 sec

distance = Vi*t/2 = 75/3.6*4.34/2 = 45.2 m

reaction time = 1000*(48-45.2)*3.6/75 = 134 msec

- AshLv 75 months ago
initial velocity, u = 75 km/h x (1000m/1km)x(1h/3600s) = 20.83 m/s

braking distance, s =(v²-u²)/2a = [0²-(20.83)²]/(2*(-4.8)) = 45 m

After noticing the stalled vehicle, you have 48-45 = 3 m to travel

time = distance / speed = (3m)/(20.83 m/s) = 0.14 s

You have just 0.14s to hit the brakes