# Homework Help: Hi, im having trouble finding out what formula i should use for this physics question

1. Apr 15, 2007

### ImsoFly

1. The problem statement, all variables and given/known data
Swati was driving her car over the speed limit when an oncoming car car flashes its lights warning her of a radar trap ahead. She decelerates from 39 m/s to 27 m/s in 6.0 s. What deceleration did her brakes apply?

- im not sure what formula to use so does anyone have a clue what formula i could use for this question? Thanks for taking your time.

2. Apr 15, 2007

### cristo

Staff Emeritus
Well, what's the definition of acceleration (or in this case deceleration) that you have come across in your lessons, or in your textbook?

3. Apr 15, 2007

### ImsoFly

umm well my teacher taught us this new displacement formula but i dunt think its related to the question i need help with .. but this is the formula he taught us: d= v1 deltat+1/2 a(deltat)2 sorry i dunt kno how to make the traingle for delta and the arrow heads... im new to this

4. Apr 15, 2007

### ImsoFly

but we usually use the average velocity=delta d/delta t formula..

oh..i fthink its this formula a=delta t + v2-v1 is this correct?

Last edited: Apr 15, 2007
5. Apr 15, 2007

### KingNothing

$$$a = {\textstyle{{\Delta v} \over {\Delta t}}}$$$

6. Apr 15, 2007

### ImsoFly

thanks KingNothing

7. Apr 15, 2007

### ImsoFly

But if i was to use a=delta v/delta t .. how do i put to veclotiys? 39 m/s to 27 m/s

8. Apr 15, 2007

### cristo

Staff Emeritus
$\Delta v$ means "change in velocity" just like $\Delta t$ means change in time. Here, the change in time is 6 seconds, and the change in velocity is exactly what it sounds like it should be; namely (39-27)m/s.

9. Apr 15, 2007

### ImsoFly

oh so would i just do 39-27? which is 12 m/s?

10. Apr 15, 2007

### cristo

Staff Emeritus
Yup, then substitute into the equation for acceleration.

11. Apr 15, 2007

### ImsoFly

Alright, thanks alot cristo for ur help and KingNothing for the formula. Thanks.