Velocity with negative acceleration!

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1. Oct 14, 2015

PhysicsBoyMan

1. The problem statement, all variables and given/known data
Train goes 20 m/s
Brakes cause a = -1 m/s2
Displacement at t = 40s ?

2. Relevant equations
v = 20 m/s
a = -1 m/s2
t = 40s

3. The attempt at a solution

It seemed like this formula fit perfectly. It had all of the known variables and the one unknown neatly out front.

The velocity is too fast and the deceleration too slow for the displacement to be 0m.

The only thing I could think of was that my delta x should have data but the initial position is 0 and the final position is unknown.

I can't imagine where such a simple problem went wrong.

2. Oct 14, 2015

AndrewHolowack

If every second the train is going 1 m/s slower than it was the previous second how fast is it going after 20 seconds? Does that help?

3. Oct 14, 2015

PhysicsBoyMan

Is it really that simple though?

The first part of my equation is displacement = velocity x time without regard for acceleration. Before deceleration the train would go 800m.

I'm not sure how to represent your idea mathematically. Going -1 m/s2 x 40s doesn't seem right to me.

4. Oct 14, 2015

AndrewHolowack

Your equation is right, and so is your answer if you assume that the negative acceleration continues for the entire 40 seconds. At 20 seconds the train comes to a stop and then begins to move backwards. At 40 seconds the train is back were you began with a velocity of -20 m/s. They way the question is worded you could argue that the train will simply come to a stop at 20 seconds.