Angular speed

[mspeedwagon]

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I'm basically summarizing the question here (there are graph choices, aim is to select best graph, but don't have these electronically).

A person does ONE bicep curl with a dumbbell starting with arm fully extended and straight down and ending back in this position. Select a graph that best describes angular speed of this motion vs time.

What should the above graph look like?

I think this would be parabolic (change in angle gets to some maximum and then decreases), but I'm not certain. Explanations welcome. Thanks.

 

[milski]

Something resembling a sine wave would be your best bet. The angular velocity starts from zero, increases to some number and then decreases to zero again when the person reaches the highest point. From there, the AV becomes negative as the arm starts moving in the opposite direction, reaches some minimum and comes back to 0 once the arm is at rest again. The shape can be not as smooth, but you want the graph to start at , have a positive first half, cross zero again and have negative second half before coming to 0 at the end.
A parabola or something similar can work for the angle itself but not for the velocity, since it does not account for the negative velocity during the return motion.

 

[mspeedwagon]

I agree regarding angular velocity being a sine wave, but question says angular speed. Wouldn't this be different from angular velocity?

Something resembling a sine wave would be your best bet. The angular velocity starts from zero, increases to some number and then decreases to zero again when the person reaches the highest point. From there, the AV becomes negative as the arm starts moving in the opposite direction, reaches some minimum and comes back to 0 once the arm is at rest again. The shape can be not as smooth, but you want the graph to start at , have a positive first half, cross zero again and have negative second half before coming to 0 at the end.
A parabola or something similar can work for the angle itself but not for the velocity, since it does not account for the negative velocity during the return motion.

 

[milski]

I agree regarding angular velocity being a sine wave, but question says angular speed. Wouldn't this be different from angular velocity?

Sorry, I must have misread the question. If it's strictly about speed, speed is the size of the velocity. All you have to do is mirror the negative parts of the graph above the x-axis. In other words, you will end up with two parabolas next to each other. There is a point at the top of curl where the arm reverses direction - the velocity and speed will have to go through zero at that point.

 

[mspeedwagon]

That's what I thought. I think the book I'm using is a little loose on speed vs velocity (MCAT books seems not to be stringent as physics textbooks). It has the sine wave graph selected as the correct answer for angular speed (though in my book that's angular velocity). Thanks for confirming.

Sorry, I must have misread the question. If it's strictly about speed, speed is the size of the velocity. All you have to do is mirror the negative parts of the graph above the x-axis. In other words, you will end up with two parabolas next to each other. There is a point at the top of curl where the arm reverses direction - the velocity and speed will have to go through zero at that point.