# Homework Help: Kinematics in a straight line

1. Oct 22, 2006

### ellusion

hi im new to this and i have a problem that im stumped on.

Two trains, one travelling at 100km/h and the other at 128km/h, are headed towards one aanother along a straight level track. When the trains are 1.2km apart, each engineer sees the other train and applies the brakes. Both trains have equal, constant decelerations of 0.9 m/s^2. What is the distance they will be apart?

2. Oct 22, 2006

I suggest you first write down the equations for displacement in the case of constant acceleration (i.e. deceleration), and present us some attempts.

3. Oct 22, 2006

### ellusion

well this is what i have,

d=vt - 1/2at^2 and i believe i should find the time it takes

a= 0.9m/s^2 = 3.24km/h^2

Train 1
1.2 = 100t - 1/2(3.24)t^2
0= -1.2 + 100t - 1.62t^2

then i use the quadratic eqn to solve for t. am i on the right track?

4. Oct 22, 2006

Calculate the time it takes for every train to stop separately out of the equation $$v(t) = v_{0} - at = 0$$. Then, plug the times t into the equations for displacement for each train separately and calculate the distance each train travels until it stops. It should be fairly easy to find how far they are apart from each other now.