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

[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?

 

[kuruman]

What is the question to be answered? If you need to use radius, take the diameter and divide it by 2.

 

[eglaud]

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,

 

[kuruman]

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?

 

[eglaud]

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

 

[kuruman]

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.

 

[eglaud]

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.

 

[kuruman]

I am confused now. What is the right answer according to your homework, 8.9 m/s or 8.9 / 2 m/s?

 

[eglaud]

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.

 

[kuruman]

... 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.

 

[eglaud]

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.

 

[kuruman]

Good luck on your exam.

 

[eglaud]

Good luck on your exam.

Thank you!