# I need help with the formula pls

## Homework Statement

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

## 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!

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

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

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

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

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

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?

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

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

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

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

hage567
Homework Helper
They are given in the question.

They are given in the question.

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

hage567
Homework Helper
The question says the car comes to a stop. That means that v2=0.

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

ohh, yes i forgot about that! thanks !

but do u kno how i would do deceleration?

hage567
Homework Helper
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.

but do u kno how i would do deceleration?

deceleration = negative acceleration, that's it

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.
oh ok thnx:tongue:

deceleration = negative acceleration, that's it

oh i never knew that lol! thnx