# Why does this screwdriver roll in a curved trajectory?

I am trying my level best to give this screwdriver a horizontal push only, but it is going in a circular path. I thought of uneven friction due to non-uniform normal reaction...what do you think? I have read up torque a bit, but I dont have a clear understanding of it..does it have anything to do here?

if the above doesn't work: http://www.youtube.com/watch?v=B21BBV0PUXo&feature=youtu.be

• The blunt end of the screwdriver travels a longer distance per revolution than the sharp side. This is similar to how a cone would roll if you were to apply a torque. – Greg May 22 '13 at 16:23
• so it has to do with torque? – Saurabh Raje May 22 '13 at 16:31
• for eg., I thought, well, if I rolled a log of wood, it would go straight and would not turn, but this does turn! – Saurabh Raje May 22 '13 at 16:33
• @Greg, what do u think? – Saurabh Raje May 22 '13 at 16:57
• A log of wood has (roughly) the same diameter at both ends so the distance traveled per revolution is the same. Because the diameters of the two ends of the screwdriver are substantially different, you get a curved path, because the number of revolutions has to be equal for the screwdriver not to twist in on itself – Michiel May 22 '13 at 17:10

$$d=2\pi r$$ For $n$ number of rotations, the equation will look like:
$$d=n 2 \pi r$$ Since every part of the screwdriver is rolling at the same speed (i.e., each part of the screwdriver will have the same number of rotations (n) over a given time) the thicker a given side is the further it will travel relative to a thinner side. That is why the thicker side of the screwdriver travels a longer distance than the thinner side, giving you the circular motion you see.