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

  • Thread starter eglaud
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In summary, a discus thrower spins in a circle and releases a discus with a rotation time of 1.2 seconds and a circle diameter of 1.7 m. To solve for the velocity, the formula v = ωr is used, where ω is found by taking 2π/1.2 and multiplying by the radius. However, in this specific problem, the correct answer is found by multiplying ω by the diameter instead of the radius. This has caused confusion and incorrect answers being entered in various sources. It is recommended to bring this issue to the instructor's attention.
  • #1
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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  • #2
What is the question to be answered? If you need to use radius, take the diameter and divide it by 2.
 
  • #3
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,
 
  • #4
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?
 
  • #5
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
 
  • #6
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.
 
  • #7
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.
 
  • #8
I am confused now. What is the right answer according to your homework, 8.9 m/s or 8.9 / 2 m/s?
 
  • #9
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!
 

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

1. What is a discus thrower?

A discus thrower is an athlete who competes in the sport of discus throwing, which involves throwing a heavy disc as far as possible with a spinning motion. It is an event in track and field competitions.

2. How heavy is a discus?

The weight of a discus varies depending on the age and gender of the athlete. For men, the discus weighs 2 kilograms (4.4 pounds) and for women, it weighs 1 kilogram (2.2 pounds).

3. What is the technique used in discus throwing?

The technique used in discus throwing involves a series of movements that allow the athlete to generate maximum power and distance. This includes a wind-up, a turn, and a release of the discus at the right angle and speed.

4. What muscles are used in discus throwing?

The main muscles used in discus throwing are the core muscles, including the abdominal muscles, back muscles, and gluteal muscles. The arm and shoulder muscles are also used to generate power and control the throw.

5. What are the rules of discus throwing?

The rules of discus throwing vary slightly depending on the competition, but generally, the athlete must stay within a throwing circle, the discus must land within a designated sector, and the athlete must not touch the top of the rim during the throw. There are also specific fouls that can result in disqualification.

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