Microsoft Interview Question

[Kev]

Originally posted by: Indolent

Originally posted by: Kev
I just did a mini experiment with a bottle cap (haha) and I see what indolent is talking about

I'm majoring in mechanical engineering and have had a dynamics course or two. This question has come up in different forms.

And I'm math/science retarded, so I think I'll believe you. I haven't had physics in like 5 years, and I sucked at it then, too.

 

[TuxDave]

Originally posted by: dighn

Originally posted by: TuxDave

Originally posted by: Indolent

Originally posted by: ActuaryTm
For those interested.

thank you

I don't think there's been an official answer on that board either... it looks like they're debating it too.

well one thing's for sure, it's definitely not forward like most people here say

Yeah... I was hoping for a 'stay put' answer. I think that would've been more interesting. Ah well...

 

[Kev]

why would MS ask a physics question in an interview though?

 

[Brazen]

Originally posted by: HardcoreRobot

Originally posted by: dighn

Originally posted by: HardcoreRobot
when you tug on it, it exerts an equal and opposite force against the bike rim. (so, a vector heading to the left). this vector gets decomposed to the horz and vert components. we dont care about the horz, just the vert, which is pushing it down. this causes the wheel to move counter clockwise and roll the bike forward.

i have no idea if that is right, but i tried to make it sound convincing

but no slipping means there's a counter force at the bottom of the wheel, so it should turn clockwise and move backward

i think the deal with the slipping is, that instead of wanting to rotate, the wheel and whole bike would just move laterally across the surface instead of rolling

I think you are correct, but also while being dragged the wheel will want to rotate the string up - I suppose that depends on the amount of friction against the wheel from the ground, but the bike will definately be moving in the direction of p.

edit: just notice the "no slipping" clause so the string would definately up the wheel as the bike is pulled backward.

 

[Indolent]

Originally posted by: TuxDave

Originally posted by: Indolent

Originally posted by: ActuaryTm
For those interested.

thank you

I don't think there's been an official answer on that board either... it looks like they're debating it too.

I read it as a concensus of the bike moving backwards. All points of it moving forward seem to have been refuted. I only skimmed it though.

 

[Jzero]

The rear wheel would stay put but the front wheel would fly up off the ground.

 

[dighn]

Originally posted by: Kev
why would MS ask a physics question in an interview though?

well physics takes analytical skills which would be what you need in programming, and its just high school physics so you don' t need extensive physics background to get. makes sense to me.

 

[TuxDave]

Hey Indolent. Is there a point around the wheel that when you pull will make the wheel go forward?

 

[RagingBITCH]

I'm still confused as to why it moves backwards. Anyone have a non-technical answer for it? Does the angle you're tugging on the string at make a difference? IE, if the point at which you're holding & tugging the string at parallel to the point to where it's attached, or if you're holding it at seat level for example.

 

[Indolent]

Originally posted by: TuxDave
Hey Indolent. Is there a point around the wheel that when you pull will make the wheel go forward?

Nope because any point on the wheel is above the center of rotation.

The only point that does anything different is if you pulled on the exact point where it touches the ground. Then it wouldn't move because there is no resulting moment.

 

[TuxDave]

Originally posted by: Indolent

Originally posted by: TuxDave
Hey Indolent. Is there a point around the wheel that when you pull will make the wheel go forward?

Nope because any point on the wheel is above the center of rotation.

The only point that does anything different is if you pulled on the exact point where it touches the ground. Then it wouldn't move because there is no resulting moment.

Hmm... very educational. Thanks.

 

[Indolent]

Originally posted by: RagingBITCH
I'm still confused as to why it moves backwards. Anyone have a non-technical answer for it? Does the angle you're tugging on the string at make a difference? IE, if the point at which you're holding & tugging the string at parallel to the point to where it's attached, or if you're holding it at seat level for example.

If you were pulling the string so that it was angled down below the point of contact, it would move forward.

Not sure if I explained this very well. I'll see if I can draw a picture

 

[ActuaryTm]

Originally posted by: Kev
I just did a mini experiment with a bottle cap ...

For those wishing to experiment to some degree of accuracy, the top of a CD/DVD media cakebox would work rather well I imagine.

 

[RagingBITCH]

Originally posted by: Indolent

Originally posted by: RagingBITCH
I'm still confused as to why it moves backwards. Anyone have a non-technical answer for it? Does the angle you're tugging on the string at make a difference? IE, if the point at which you're holding & tugging the string at parallel to the point to where it's attached, or if you're holding it at seat level for example.

If you were pulling the string so that it was angled down below the point of contact, it would move forward.

Not sure if I explained this very well. I'll see if I can draw a picture

After reading your TuxDave explanation, I got it. A lot of it also depends on where the spoke is, correct? IE, if it was perpendicular to the ground it would have to move forward?

 

[TheShiz]

stay put

 

[Indolent]

Originally posted by: RagingBITCH

Originally posted by: Indolent

Originally posted by: RagingBITCH
I'm still confused as to why it moves backwards. Anyone have a non-technical answer for it? Does the angle you're tugging on the string at make a difference? IE, if the point at which you're holding & tugging the string at parallel to the point to where it's attached, or if you're holding it at seat level for example.

If you were pulling the string so that it was angled down below the point of contact, it would move forward.

Not sure if I explained this very well. I'll see if I can draw a picture

After reading your TuxDave explanation, I got it. A lot of it also depends on where the spoke is, correct? IE, if it was perpendicular to the ground it would have to move forward?

I'm not sure I understand what you mean by where the spoke is.

Here is a picture with the same idea. P1 makes it go right (backward). P2 makes it go left (forward).


*edit* oops, here

 

[Britboy]

Originally posted by: soniikboom
Why is it forward?

I think its harder than it seems.

A McDonalds employee could answer that question:roll:

 

[ISAslot]

I tried it. goes backward at just about every point on the wheel. Never forward. only pulling in the direction of P though

 

[Indolent]

Originally posted by: Britboy

Originally posted by: soniikboom
Why is it forward?

I think its harder than it seems.

A McDonalds employee could answer that question:roll:

woah, read the thread.

 

[Goosemaster]

1. Because of the infinite static friction, there is no slippage. The force of static friction points backwards, as it is not letting the wheel slide forwards.
2. The x component of force P would be much greater than its y component. The y component is what allows it to go forward without sliding.
3. The opposite and equal (somewhat equal) force has yet to be accounted for, as both P and F are in the backward direction.
4. The Torque of P is directed in the counterclockwise direction.
5. It stays put .


EDIT:

6. I would LIKE TO SAY that with an increased P, it would roll backwards, but I cannot get myself to say it even tough I want to because I can't prove it.

Let's say that it was directly behind the spoke all the way at the rim. In the case the torque would be directed at the center of the circle, which would mean that it would be serving as a torque around the point where the rim is making contact with the pavement. if enough force P was applied, i guess the y component would be great enough to move it backwards without slipping, but it would take a lot of force...

editx2:

I guess Indolent got it. I really don't know much about finding out how internal torques act on external bodies outside of a circle l (err.. I SHOULD, but I don't )

That said, at least I proved why it can;t go foward

 

[Brazen]

kudos to indolent, he explains it well and pointing out that the center of rotation would be where the wheel is in contact with the gound really makes it obvious as to why the bike will move backward. Nice drawing too.

 

[Indolent]

Originally posted by: Goosemaster
1. Because of the infinite static friction, there is no slippage. The force of static friction points backwards, as it is not letting the wheel slide forwards.
2. The x component of force P would be much greater than its y component. The y component is what allows it to go forward without sliding.
3. The opposite and equal (somewhat equal) force has yet to be accounted for, as both P and F are in the backward direction.
4. The Torque of P is directed in the counterclockwise direction.
5. It stays put .


EDIT:

6. I would LIKE TO SAY that with an increased P, it would roll backwards, but I cannot get myself to say it even tough I want to because I can't prove it.

Let's say that it was directly behind the spoke all the way at the rim. In the case the torque would be directed at the center of the circle, which would mean that it would be serving as a torque around the point where the rim is making contact with the pavement. if enough force P was applied, i guess the y component would be great enough to move it backwards without slipping, but it would take a lot of force...

I'm not really understanding what you are trying to say, but your first point is wrong. The forct of static friction points forward. I think you are assuming the wheel is rotating about it's axis when it is really rotating about the ground.


*edit* thanks Brazen. I was hoping I could explain it understandably.

 

[Goosemaster]

Originally posted by: Indolent

Originally posted by: Goosemaster
1. Because of the infinite static friction, there is no slippage. The force of static friction points backwards, as it is not letting the wheel slide forwards.
2. The x component of force P would be much greater than its y component. The y component is what allows it to go forward without sliding.
3. The opposite and equal (somewhat equal) force has yet to be accounted for, as both P and F are in the backward direction.
4. The Torque of P is directed in the counterclockwise direction.
5. It stays put .


EDIT:

6. I would LIKE TO SAY that with an increased P, it would roll backwards, but I cannot get myself to say it even tough I want to because I can't prove it.

Let's say that it was directly behind the spoke all the way at the rim. In the case the torque would be directed at the center of the circle, which would mean that it would be serving as a torque around the point where the rim is making contact with the pavement. if enough force P was applied, i guess the y component would be great enough to move it backwards without slipping, but it would take a lot of force...

I'm not really understanding what you are trying to say, but your first point is wrong. The force of static friction points forward. I think you are assuming the wheel is rotating about it's axis when it is really rotating about the ground.

Isn't it both?

The force of static friction prevents it from slipping backward, and then foward, which changes the overal axis of rotation....?

I just didn;t point of the last one because I don;t know enough about it

 

[Bassyhead]

forward

 

[Indolent]

Originally posted by: Goosemaster

Originally posted by: Indolent

Originally posted by: Goosemaster
1. Because of the infinite static friction, there is no slippage. The force of static friction points backwards, as it is not letting the wheel slide forwards.
2. The x component of force P would be much greater than its y component. The y component is what allows it to go forward without sliding.
3. The opposite and equal (somewhat equal) force has yet to be accounted for, as both P and F are in the backward direction.
4. The Torque of P is directed in the counterclockwise direction.
5. It stays put .


EDIT:

6. I would LIKE TO SAY that with an increased P, it would roll backwards, but I cannot get myself to say it even tough I want to because I can't prove it.

Let's say that it was directly behind the spoke all the way at the rim. In the case the torque would be directed at the center of the circle, which would mean that it would be serving as a torque around the point where the rim is making contact with the pavement. if enough force P was applied, i guess the y component would be great enough to move it backwards without slipping, but it would take a lot of force...

I'm not really understanding what you are trying to say, but your first point is wrong. The force of static friction points forward. I think you are assuming the wheel is rotating about it's axis when it is really rotating about the ground.

Isn't it both?

The force of static friction prevents it from slipping backward, and then foward, which changes the overal axis of rotation....?

I just didn;t point of the last one because I don;t know enough about it

I'm not sure where you are seeing a force that is trying to make the wheel slip forward. Since there is only one point of rotation and one force pulling the wheel, there is only one friction force at the contact point.