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1-Newbie

## angular acceleration

Hey, I was hoping somebody could give me some help on this question. I tried it many times but could not get it. Most likely my units are off or possibly using the wrong formula/ equation.
Here it is:

The tires of a car make 65 revolutions as the car reduces its speed uniformly from 95 km/h to 45 km/h. The tires have a diameter of 0.80m. (a) What was the angular acceleration of the tires? (b) If the car continues to deccelerate at this rate, how much more time is required for it to stop?

Okay, I changed the unif. speed into m/s, I wasn't too sure if I should, but I I changed 65 rev into 9420 rads.
Anyway I thought of using this formula
w^2= w(initial)^2 + 2*(angular accel.)*(angle)
I'm not too sure if it is the correct formula since it does not require the use of the diameter or radius. Okay, thanks for the help.
3 REPLIES 3
On 10/19/2005 10:25:48 PM, mdao wrote:
>Hey, I was hoping somebody
>could give me some help on
>this question. I tried it
>many times but could not get
>it. Most likely my units are
>off or possibly using the
>wrong formula/ equation.
>Here it is:
>
>The tires of a car make 65
>revolutions as the car reduces
>its speed uniformly from 95
>km/h to 45 km/h. The tires
>have a diameter of 0.80m. (a)
>What was the angular
>acceleration of the tires?
>(b) If the car continues to
>deccelerate at this rate, how
>much more time is required for
>it to stop?
>
>Okay, I changed the unif.
>speed into m/s, I wasn't too
>sure if I should, but I I
>changed 65 rev into 9420 rads.
>Anyway I thought of using this
>formula
>w^2= w(initial)^2 + 2*(angular
>accel.)*(angle)
>I'm not too sure if it is the
>correct formula since it does
>not require the use of the
>thanks for the help.

Why that one? This should be the same as if the velocities were linear velocities

distance traveled = wi+acc*t2/2

and wf = wi + acc*t

substituting for acc*t in the first equation allows you to find the time duration of the deceleration. In order to figure out the angular velocity, you need to use the radius.

TTFN,
Eden
Your formula is appropriate, as it relates quantities that you have, or can easily calculate, to the desired quantity, angular acceleration. This formula is purely in terms of angles, and so does not involve any consideration of radii or diameters. But you will need the radius or diameter to convert the given linear velocities into angular velocities, as needed for the formula.

You can use revolutions or radians as your angular unit, as you wish. But you have to be consistent about it. And be careful about using formulae relating angular and linear units. Because angles are typically considered to be dimensionless, such formula often imply a particular angular unit, and it is up to you to know what angular unit is implied.

But how did you convert your revolutions into radians? That value looks to be way off. How many radians in a revolution?

� � � � Tom Gutman
hey, yeah thanks that helped and I go the answer. their are 2*(pi) or 2*3.14 radians in a revolution.
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