# I need help with the formula pls

1. Apr 17, 2007

### ImsoFly

1. The problem statement, all variables and given/known data
When a car is being tested, it also has a braking test done/ In one such test a car took 3.0 s to stop from 40 m/s. Calculate a) the braking distance b) the deceleration

3. The attempt at a solution
first off i'm not sure how to find the braking distance and what formula to use, and also for deceleration is there a different formula than d=vxt..v=d/t...etc? If so please show me it thanks for taking your time!

2. Apr 17, 2007

### Murdoc88

Do you know of the Uniform Acceleration formulas?

$a = v / t$

$d = [(V1+V2) / 2 ]t$

$d = (V1)(t) + 1/2(a)(t^2)$

$d = (V2)(t) - 1/2(a)(t^2)$

$(V2)^2 = (V1)^2 + 2(a)(d)$

where:

a = Acceleration
d = distance
V1 = Initial velocity
V2 = Final velocity
t = time

3. Apr 17, 2007

### ImsoFly

yes , i know those formula but thanks for pointing those out:tongue:
But i'm a little confused on how to find the deceleration!

4. Apr 17, 2007

### Murdoc88

well once you find 'd' you will know be able to use either the 3rd or 4th and get accelleration, which should be negative

5. Apr 17, 2007

### ImsoFly

umm ok so to calculate a) the braking distance...would i use the a= delta v/delta t formula? If so then what do i use next?

6. Apr 17, 2007

### Murdoc88

not quite, try using one of those five .. maybe #2. remember final velocity is 0 m/s

7. Apr 17, 2007

### Murdoc88

once you find distance you can find acceleration, which in this case is deceleration.

8. Apr 17, 2007

### ImsoFly

if so, im not sure how i would get the v1 and v2

9. Apr 17, 2007

### hage567

They are given in the question.

10. Apr 17, 2007

### ImsoFly

v1 and v2 : velocity theres only seconds and one velocity...

11. Apr 17, 2007

### hage567

The question says the car comes to a stop. That means that v2=0.

12. Apr 17, 2007

### ImsoFly

ohh, yes i forgot about that! thanks !

13. Apr 17, 2007

### ImsoFly

but do u kno how i would do deceleration?

14. Apr 17, 2007

### hage567

I'm not sure what you mean. You just use

$$a_{ave} = \Delta v / \Delta t$$

where $$\Delta v$$ = v2-v1. You were given the time interval in the question. Since it is deceleration, you will get a negative number.

15. Apr 17, 2007

### Feldoh

deceleration = negative acceleration, that's it

16. Apr 17, 2007

### ImsoFly

oh ok thnx:tongue:

17. Apr 17, 2007

### ImsoFly

oh i never knew that lol! thnx