The discussion revolves around the concept of angular acceleration, particularly in the context of an object rotating clockwise and slowing down. Participants explore the relationship between angular speed, angular acceleration, and the conventions used to define their directions.
Participants express varying interpretations of the relationship between angular acceleration and the direction of rotation. While some agree on the conventions of positive and negative directions, others remain uncertain about the implications of these conventions in specific scenarios, leading to multiple competing views.
Participants reference different conventions for defining angular speed and acceleration, which may lead to confusion. The discussion also highlights the need for clarity regarding the terms used, such as "decelerating" versus "slowing down."
This discussion may be useful for students or individuals interested in understanding angular motion, particularly those grappling with the concepts of angular acceleration and the conventions used in physics.
Doc Al said:What's the direction of the change in angular speed?
Another example, for linear motion: Toss a ball straight up into the air. It's moving up and slowing down, but what's the direction of its acceleration?
Right.janettaywx said:the gravitational acceleration is always coming down.
so the ball's accel is downwards?
Right.janettaywx said:hmm. wait. say i define positive upwards, g is negative cos it is downwards.
The ball is moving upwards, but slowing down. So its acceleration is negative (downwards).the ball is deccelerating upwards so it is negative?
So which direction is its acceleration? Clockwise or counterclockwise? Then you can decide whether it should be called positive or negative.janettaywx said:but how abt an object rotating clockwise is slowing down?
The usual convention is that counterclockwise is positive and clockwise is negative.when it rotates clockwise, its angular speed is negative right?
i not sure the direction of angular speed as it is not stated in my notes.
This is true.my notes just say " angular acceleration is positive if object rotating counterclockwise is speeding up and angular acceleration is positive if object rotating clockwise is slowing down"
Ask yourself:janettaywx said:hmm. why " angular acceleration is positive if object rotating counterclockwise is speeding up and angular acceleration is positive if object rotating clockwise is slowing down" is true?
Doc Al said:Ask yourself:
A) If something rotating counterclockwise speeds up, which way is it accelerating? Clockwise or counterclockwise?
B) If something rotating clockwise slows down, which way is it accelerating? Clockwise or counterclockwise?
Then use the sign convention to determine whether it's positive or negative.
Sounds good, but with some comments. Careful with the term "decelerating"--I find it more confusing than helpful. (It just means "slowing down".) Much better to stick with acceleration, which can be positive or negative. So for B I would say it's moving clockwise but slowing down (decelerating), thus the acceleration is counterclockwise and thus positive.janettaywx said:A) it is accelerating counterclockwise as well so it is positive?
B) it is decelerating in clockwise direction (negative). which means it is accelerating in counterclockwise direction and so it is positive as well?