# Help with kinematic equations.

1. Sep 28, 2007

### pebbles

hello,
i'm having trouble with kinematic equations. i feel that i understand them, but when i get to a problem, i'm so confused. i can figure out what i have and what i don't have, but it's choosing the correct formula that is my main problem.

how do i know when to solve for what?

for example, i'm given speed of 25 m/s [acceleration is constant] and i know that the object comes to rest after 34 seconds and now i am asked to solve for distance

initial speed=25 m/s, final speed=0, time=34 s.

would i use x=1/2(final velocity + initial velocity)t?

2. Sep 28, 2007

### pebbles

what is constant acceleration, zero?

3. Sep 28, 2007

### Link-

Yes, you should use equation $$\Delta$$x=1/2(Vf+ Vi)t

And not, constant acceleration is not zero. Acceleration is change in velocity, and there's change in velocity since the final speed is 0. Constant acceleration means that velocity change with respect to time is constant.

a=$$\Delta$$ V /$$\Delta$$ t or more specific a=dv/dt.

Here's an example. If you are in a car and you push the gas pedal your car will change velocity (accelerate), if you push further and further the gas pedal, your acceleration would change.
If your change in velocity with respect to time is constant then you have constant acceleration.

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