# Physics Help!

1. Jul 27, 2006

### wells=mvp

Hi, I just had a question i had trouble answering, im sure its an easy one, but i cant get it, (since im just starting physics):

a bus starts at rest and accelerates at 1.5m/s^2 until it reaches a velocity of 9m/s. The bus continues at this velocity and then decelerates at -2m/s^2 until it comes to a stop 400m from its starting point. How much time did the bus take to cover the 400m? (ans: 50 seconds)

Im using basic velocity and acceleration formulas and I cant get the answer.

2. Jul 27, 2006

### Pythagorean

show us how you work the problem. Have you ever used latex?

Last edited: Jul 27, 2006
3. Jul 27, 2006

### sdekivit

there are three types of movement: the acceleration phase, the movement woth constant velocity and the decceleration.

1.To calculate the time and distance during the acceleration and decceleration use:

$$a = \frac {\Delta v} {\Delta t}$$

--> distance at these phases: $$s = \bar{v} \cdot t$$ with

$$\bar{v} = \frac {1} {2} (v_{2} - v_{1})$$

2. To calculate the time of the remaining distance with constant velocity use $$s = v \cdot t$$

3. Then sum the calculated times to get to the answer: $$t = t_{acc} + t_{cst. v} + t_{dec}$$

Last edited: Jul 27, 2006
4. Jul 27, 2006

### Hootenanny

Staff Emeritus
Alternatively, one could use equations of uniform acceleration.