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Microsoft Interview Question

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Originally posted by: soniikboom
Why is it forward?

I think its harder than it seems.
Click to expand...

my answer would be foward assuming that the bike stood up straight by itself and the force of pulling the string was enough set the wheel spenning counter-clockwise.

If the bike fell down, it would move backward of course.


The questions are more to see how you think than anything. Of course you gotta get the right answer too.
 
looks like backwards to me... force to the right on the point of contact with string and force to the left on the bottom of the wheel should turn the wheel back
 
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
 


What if instead of just simply pulling the string...a man holding the string pulled it up to his chest. How would this affect the movement? I think this was actually the question I got.
 
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
Click to expand...

but no slipping means there's a counter force at the bottom of the wheel, so it should turn clockwise and move backward
 
Forward, you are moving the point where the string is attached to the wheel closer to the person when the string is pulled. This means that the center of the wheel (the hub) must move further away, and so does the rest of the bike.
 
how do we know P is going in the "right" direction? i mean what if it is coming along the Z axis at you in some way? then the bike owuld just tip over and go nowhere 😀

//no one ever said the pic was in 2d
 
Originally posted by: mzkhadir
I think the bike would move forward, but I think it has something to do with the force of the string pull.

because of the direction of the pull. It would not go backwards because the wheel is spinning forward and the bike will go forward.
Click to expand...

Winner.
 
There are two points of rotation. The first is the rotation about the axle of the wheel and the second point is at the point of contact between the wheel and ground.
 
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
Click to expand...

but no slipping means there's a counter force at the bottom of the wheel, so it should turn clockwise and move backward
Click to expand...
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 honestly have a hunch that if there is guaranteed no slipping, then it stays put. Anyone want to back me up?
 
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
Click to expand...

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

it means that the friction causes the bike to not remain stationery when the wheel spins.
 
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
Click to expand...

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


Correct, it will move backwards. The center of rotation is the point of contact between the wheel and the ground. Since the force is higher, it will move it backwards. No slip conditions is the key.
 
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