# Homework Help: Rotational Kinematics

1. Jan 12, 2010

### nahanksh

1. The problem statement, all variables and given/known data
https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys211/spring09/homework/10/bowling_ball/8.gif [Broken]
A bowling ball 25 cm in diameter is slid down an alley with which it has a coefficient of sliding friction of ยต = 0.6. The ball has an initial velocity of 11 m/s and no rotation. g = 9.81 m/s^2.
Given that the initial deceleration of the ball is 5.886.

What is the initial angular acceleration of the ball?

2. Relevant equations
For a sphere Icm = (2/5)mr^2.

3. The attempt at a solution

Firstly, i tried to use the formula $$\alpha = a/R$$
Then i got some value of acceleration which turns out to be wrong.

After that, when i used the torque equation $$\tau = I\alpha$$
I got the different answer and it was correct...

Why did i get the wrong answer at the first attempt?
I am really confused when i could use the transformation formulae($$s to \theta, v to \omega, a to \alpha$$)....

Could someone help me out here..?

Last edited by a moderator: May 4, 2017
2. Jan 12, 2010

### Gear300

When you used $$\alpha = a/R$$, I'm assuming the acceleration you used was the deceleration value they gave you. That value (the deceleration value) is the translational deceleration, meaning it is the deceleration of the center of mass of the ball. The equation you used is referring to the tangential acceleration of a point on the ball that is a radial distance R from some reference point (which you probably took as the center of mass).

Last edited: Jan 12, 2010