Why do we use diameter instead of radius to find velocity in discus throwing?

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Homework Help Overview

The discussion revolves around a physics problem related to the motion of a discus thrower, specifically addressing the calculation of velocity using the diameter of the circular path instead of the radius. The problem provides the diameter of the circle and the time taken for one rotation.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between angular velocity and linear velocity, questioning why the diameter is used in the calculation instead of the radius. Some express confusion over the correct application of formulas and the validity of the answers obtained.

Discussion Status

The discussion is ongoing, with participants seeking clarification on the use of diameter versus radius in the context of the problem. Some have provided insights into the calculations, while others express uncertainty about the correctness of the answers and the potential for errors in provided solutions.

Contextual Notes

There is mention of discrepancies between homework answers and textbook solutions, leading to confusion about the correct approach. Participants are also considering the implications of these inconsistencies on their understanding and upcoming assessments.

eglaud

Homework Statement


A discus thrower spins in a circle and releases a discuss. This rotation takes 1.2 seconds, and the diameter of the circle he spins in is 1.7 m.

To solve this, I first found w by taking 2pi/1.2 to get 5.24 rads/second. Then, I multiplied this by the radius to arrive at the velocity, since w=v/r. This however, does not get me the correct answer - what does, though, is multiplying the w by the diameter to find v. I have no idea why, and every version of this on chegg uses a different equation that we haven't learnt. Why do we use diameter, instead of radius?
 
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What is the question to be answered? If you need to use radius, take the diameter and divide it by 2.
 
kuruman said:
What is the question to be answered? If you need to use radius, take the diameter and divide it by 2.

My question is the answer is found using diameter, but I cannot think of a reason why. I used the equation mr=v to find v, I have no idea why diameter is used,
 
Before I can answer your question, I need to understand the question that the problem is asking. What does the problem want you to solve for?
 
kuruman said:
Before I can answer your question, I need to understand the question that the problem is asking. What does the problem want you to solve for?
Oh sorry, the question gave me the diameter and time it took to speed in a circle to release the discus, and is asking for the velocity. I'm solving for the velocity, the answer 8.9
 
OK, I see now. You have correctly found that ω = 5.23 rad/s. To get the speed, you need to multiply by half the diameter, v = ω r. The correct answer can't be 8.9 m/s. Someone keyed the correct answer incorrectly. It happens some times.
 
kuruman said:
OK, I see now. You have correctly found that ω = 5.23 rad/s. To get the speed, you need to multiply by half the diameter, v = ω r. The correct answer can't be 8.9 m/s. Someone keyed the correct answer incorrectly. It happens some times.
It's the right answer on my homework, but I thought it had to be wrong. I did the same problem in the book, which had different numbers, but still followed the same formula to get the same (what I think is incorrect as well) answer.
 
I am confused now. What is the right answer according to your homework, 8.9 m/s or 8.9 / 2 m/s?
 
kuruman said:
I am confused now. What is the right answer according to your homework, 8.9 m/s or 8.9 / 2 m/s?

The "correct" answer is 8.9, the answer in the book is a different answer to the same problem that uses different numbers, so it just reinforced using the diameter as the radius to solve the problem. The online homework is paired to the book however, but I still think they wouldn't just plug it in using the same equation they're using for it.
 
  • #10
eglaud said:
... but I still think they wouldn't just plug it in using the same equation they're using for it.
It appears that whoever keyed in the solutions in all those venues read "diameter" but internalized "radius" and went on autopilot putting in incorrect answers everywhere. If I were you, I would take this to my instructor who needs to know that there is an issue here.
 
  • #11
kuruman said:
It appears that whoever keyed in the solutions in all those venues read "diameter" but internalized "radius" and went on autopilot putting in incorrect answers everywhere. If I were you, I would take this to my instructor who needs to know that there is an issue here.

Hopefully it is! My exam is tomorrow so I was hoping to get this confirmed by tonight haha.
 
  • #12
Good luck on your exam.
 
  • #13
kuruman said:
Good luck on your exam.
Thank you!
 

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