Motion problem: motorist's braking time including reaction time

[chemie726283]

Homework Statement

A motorist is traveling at 10m/s in a car whose maximum deceleration rate is 5.0 m/s^2

The effective reaction time between seeing a hazard and starting to brake is 0.5s

1. Calculate the thinking distance for the drover ar this speed (ie the distance traveled during his reaction time)
2. Calculate the minimim braking distance (ie the distance traveled during maximum deceleration)
3. the total minimum stopping distance at 10 m/s
4. The car approaches traffic lights at 12 m/s and the lights turn red when it is 20m from the line. Show whether it is possible for the car to stop before reaching the line and calculate how far from the line it will stop.

Homework Equations

The Attempt at a Solution

1. s= 10 x 0.5 = 5m
2. s= 10 x 2 x 1/2 x (-5) -> s= 10 m

I am stuck on 3. and 4. and don't know where to start

 

[haruspex]

s= 10 x 2 x 1/2 x (-5) -> s= 10 m

10m is right, but I don't understand how you get 10 x 2 x 1/2 x (-5), or how that becomes 10. Looks like -50 to me.

I am stuck on 3.

In part 1 you found the distance traveled before starting to decelerate, and in part 2 the distance while decelerating. Part 3 asks for the total.