1. ## Kinematic #3

Can someone help me with this problem. Thank you very much! Here is what I did so far.

Question: A train is traveling at 100 m/s. The engineer applies the brake b/c he sees a bike on the tracks ahead. The brakes caused a acceleration of -2.2 m/s^2. The enginner applies the brake when the train is 960 m from the bike. How long does it take the train to reach the bike?

Here is what I got so far...
it is asking for time, so I got the equation:
x = x not + v not t + 1/2 a t^2
= 960 + 100t + 1/2 (-2.2)t^2
I use -b +/- squareroot b^2 -4ac / 2a and got a huge number which is incorrect.

2. Originally Posted by jsu03
Can someone help me with this problem. Thank you very much! Here is what I did so far.

Question: A train is traveling at 100 m/s. The engineer applies the brake b/c he sees a bike on the tracks ahead. The brakes caused a acceleration of -2.2 m/s^2. The enginner applies the brake when the train is 960 m from the bike. How long does it take the train to reach the bike?

Here is what I got so far...
it is asking for time, so I got the equation:
x = x not + v not t + 1/2 a t^2
= 960 + 100t + 1/2 (-2.2)t^2
I use -b +/- squareroot b^2 -4ac / 2a and got a huge number which is incorrect.
$\Delta x = v_0t - \frac{1}{2}at^2$

$960 = 100t - 1.1t^2$

$1.1t^2 - 100t + 960 = 0$

$t = \frac{100 \pm \sqrt{(-100)^2 - 4(1.1)(960)}}{2(1.1)}$

two solutions for t ... choose the one that makes sense in the context of the problem.

3. Originally Posted by jsu03
Can someone help me with this problem. Thank you very much! Here is what I did so far.

Question: A train is traveling at 100 m/s. The engineer applies the brake b/c he sees a bike on the tracks ahead. The brakes caused a acceleration of -2.2 m/s^2. The enginner applies the brake when the train is 960 m from the bike. How long does it take the train to reach the bike?

Here is what I got so far...
it is asking for time, so I got the equation:
x = x not + v not t + 1/2 a t^2
= 960 + 100t + 1/2 (-2.2)t^2
I use -b +/- squareroot b^2 -4ac / 2a and got a huge number which is incorrect.