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Physics Problem Help

The time shown on a clock changes from 4:00 to 4:30 . The minute hand, of length 25cm , moves smoothly halfway around the face. The movement of the tip of the minute hand can be thought of as lots of small displacement vectors taking the tip from the old to the new position.

For the tip of the minute hand, what is the average downward displacement of the tip, over the half hour, from its position at 4:00 ?

This probably looks like it involves integration, but I have no idea how to go about it/set it up. Please could someone walk me through the steps to the answer. Thanks.
Original post by NotRike
The time shown on a clock changes from 4:00 to 4:30 . The minute hand, of length 25cm , moves smoothly halfway around the face. The movement of the tip of the minute hand can be thought of as lots of small displacement vectors taking the tip from the old to the new position.
For the tip of the minute hand, what is the average downward displacement of the tip, over the half hour, from its position at 4:00 ?
This probably looks like it involves integration, but I have no idea how to go about it/set it up. Please could someone walk me through the steps to the answer. Thanks.

Which year are you in, if you don't mind me asking?? Depending on that, I can give a valid answer :smile:
(edited 1 month ago)
Reply 2
Original post by NotRike
The time shown on a clock changes from 4:00 to 4:30 . The minute hand, of length 25cm , moves smoothly halfway around the face. The movement of the tip of the minute hand can be thought of as lots of small displacement vectors taking the tip from the old to the new position.
For the tip of the minute hand, what is the average downward displacement of the tip, over the half hour, from its position at 4:00 ?
This probably looks like it involves integration, but I have no idea how to go about it/set it up. Please could someone walk me through the steps to the answer. Thanks.

Sounds like isaac physics? If so, which one? But average is the total vertical displacement / total time and both parts of the fraction should be a simple write down?

It sounds more like a vector (cartesian) question rather than a calculus question, so some simple reasoning should be all thats required.
(edited 1 month ago)
Original post by NotRike
The time shown on a clock changes from 4:00 to 4:30 . The minute hand, of length 25cm , moves smoothly halfway around the face. The movement of the tip of the minute hand can be thought of as lots of small displacement vectors taking the tip from the old to the new position.
For the tip of the minute hand, what is the average downward displacement of the tip, over the half hour, from its position at 4:00 ?
This probably looks like it involves integration, but I have no idea how to go about it/set it up. Please could someone walk me through the steps to the answer. Thanks.

You would probably use circular motion for this right? You can calculate angular frequency by 2pi/T, where T=60x60, then calculate v=r x omega. You would then integrate v w.r.t t from t=0 to t=30x60. I don’t think this the right way of solving it.
Original post by chinnu30306
You would probably use circular motion for this right? You can calculate angular frequency by 2pi/T, where T=60x60, then calculate v=r x omega. You would then integrate v w.r.t t from t=0 to t=30x60. I don’t think this the right way of solving it.

If you were to think about vectors in terms of polar coordinates, you would end up with v pointing in the phi hat direction (take r hat to point outwards from the centre of the clock, along the minute hand; phi hat points in the clockwise direction, and k hat points into the clock face). This doesn’t sound promising because you would then have to integrate phi hat (because it too changes with time, along with r hat)

*r hat, phi hat and k hat are unit vectors along those directions
Original post by chinnu30306
If you were to think about vectors in terms of polar coordinates, you would end up with v pointing in the phi hat direction (take r hat to point outwards from the centre of the clock, along the minute hand; phi hat points in the clockwise direction, and k hat points into the clock face). This doesn’t sound promising because you would then have to integrate phi hat (because it too changes with time, along with r hat)
*r hat, phi hat and k hat are unit vectors along those directions

I just realised something- you can convert phi hat back to Cartesian coordinates once you’re done with all the cross products involved with calculating v. So now, you can easily integrate the velocity vector to get displacement. The whole thing with AVERAGE displacement can be dealt with by dividing the integral by the time period I believe (30 mins in this case)
Reply 6
Original post by chinnu30306
I just realised something- you can convert phi hat back to Cartesian coordinates once you’re done with all the cross products involved with calculating v. So now, you can easily integrate the velocity vector to get displacement. The whole thing with AVERAGE displacement can be dealt with by dividing the integral by the time period I believe (30 mins in this case)

I really think youre overcomplicating it. Just draw a clock at 4 and 4.30 and measure the vertical displacement of the minute hand and divide by 30.
Reply 7
Original post by mqb2766
I really think youre overcomplicating it. Just draw a clock at 4 and 4.30 and measure the vertical displacement of the minute hand and divide by 30.

I tried that, but 50/30 was wrong. No idea what the correct answer is still. Any help?
Reply 8
Original post by chinnu30306
I just realised something- you can convert phi hat back to Cartesian coordinates once you’re done with all the cross products involved with calculating v. So now, you can easily integrate the velocity vector to get displacement. The whole thing with AVERAGE displacement can be dealt with by dividing the integral by the time period I believe (30 mins in this case)

Thanks, but I think this is way to complicated for this problem.
Reply 9
Original post by NotRike
I tried that, but 50/30 was wrong. No idea what the correct answer is still. Any help?

Have you got a link to the question?
Reply 10
Original post by mqb2766
Have you got a link to the question?

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Original post by NotRike
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That was ridiculous, honestly.

The question asks for the average vertical displacement during that time. So what is that? Dont bother thinking about circles, just think about how the verical displacement changes and the answer is bleeding obvious. Note it does help to post a link to the question in the op as sometimes its necessary to play with things to see what theyre asking.
(edited 1 month ago)
Reply 12
Okay so apparently average displacement = median displacement.
So the displacement (vertical) of the minute hand goes from 0 --> 50cm, so the "average" is 25cm.
Original post by NotRike
Okay so apparently average displacement = median displacement.
So the displacement (vertical) of the minute hand goes from 0 --> 50cm, so the "average" is 25cm.

Thats not really the definition, but its an easy symmetry argument to make that they must be the same here and the answer is a "write down".

You should properly argue that the average vertical displacement going from t=0 to t=30 (minutes) here is
E(25 - 25*cos(theta))
where the expected value is taken from theta = 0 to pi, so the argument goes from 0 to 50. So the expected value for this question is the integral from 0 to pi of that expression and then divided by pi.

But its easier to make the symmetry argument and write down the ans.
(edited 1 month ago)

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