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A red train traveling at 72 km/hr and a green train traveling at 144 km/hr are headed toward one another along a straight level track. When they are 950 meters apart, each enginneer sees the other's train and applies the brakes. The brakes decelerate each train at the rate of 2.0 m/s^2. Is there a colllision? If so, what is the speed of each train at impact? If not, what is the separtion between the trains when they stop?

Answer :

first, convert all the km/hr into m/s

You will get that
initial speed = 20 m/s
Initial speed of Green train = 40 m/s
Initial separation = 950 m
Velocity of approach =  20 - -40 = 60 m/s
relative acceleration = -4 m/s^2

v = u + at
0 = 60 - 4t

t = 15s

s = ut + 1/2  *at * t

s = 60 * 15  - 1/2 *4 * 225
s = 900 - 450

Separation when they stop  = 450 m

hope this helps

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