# How to get the acceleration in the second and third interval please

1. Feb 18, 2006

### angina1

Can somebody explain to me how to get the acceleration in the second and third interval please.

A car starts from rest and accelerates uniformly for 10.0s, reaching a speed of 20.0m/s. It then continues traveling at that speed for 40.0 additional seconds before decelerating uniformly and coming to rest after another 25.0 seconds. Calculate the acceleration on the car during each of the three intervals and the distance traveled during each of the three intervals and the distance traveled during each time.

If acceleration formula is vf-vinitial/t. Im not sure but i think the v initial for the second interval is 20.0m/s and vf is 0 and time is 40.0 s. Can somebody tell me if I am wrong or right

Last edited: Feb 18, 2006
2. Feb 18, 2006

### dicerandom

Since these are all constant accelerations you can just calculate the average acceleration, which will be equal to the instantaneous acceleration:

$$a=\frac{v_f - v_i}{\Delta t}$$

So, for each of the three intervals just subtract the initial velocity from the final one and divide by the length of the time interval. You can also think of this graphically, if you plot the velocity versus time on a graph the slope of the lines will be the acceleration and the area under the graph will be the distance traveled.

3. Feb 18, 2006

### angina1

I know the formula, im just confuse understanding the problem, I know in the first interval the vf is 20.0 an the vi is 0 the t is 10.o so a=2.0, but in the second interval i am confused about the values that i am supposed tu use.

4. Feb 18, 2006

### nrqed

In your second interval, the question states that the car continues to travel at that speed (20 m/s) for the entire interval of 40 s. Therefore vf=vi= 20 m/s. So the accelaration is...

Pat

5. Feb 18, 2006

### angina1

I know the vo is 20.0 and the time is 40.0 but i am not sure about the vf is it 0??

6. Feb 18, 2006

### nrqed

If the question says that the car continues to travel at that speed during that interval it means that the speed does not change. That means that vf is also 20 m/s!!

7. Feb 18, 2006

### angina1

ohhh ok thank you so much for your help!!

8. Feb 18, 2006

### HallsofIvy

Staff Emeritus
The car goes from 0 to 20 mps in 10 seconds so you can alculate the acceleration during that time. It then continues traveling at that speed!!! The speed is still 20 mps at the end of the time and the acceleration is obvious!

The car then goes from 20mps to 0 in 25 s.