The common screws which we use, are right handed helices, the simplest parametric equations of which are:- $$x(s)=\cos(s),y(s)=\sin(s),z(s)=s$$, with $z$-axis as the axis of the helix.

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My question is:-

Why does a screw move forward or backward (forward for clockwise rotation of a right-handed helix), on applying a pure rotational torque?

Even if we provide only torque for rotation, i.e. force perpendicular to the axis, it moves along the axis. What force acting on it propels it forward? if it is a component of the normal reaction of the material it is driven into, what is the action-reaction pair for this normal reaction?

  • $\begingroup$ when a ring is spun around pencil it moves up. it is crude example of angular momentum. There is a toy which you move the fan blades up the helix its rotation increase thats also angular momentum. $\endgroup$ – user28737 Nov 5 '13 at 6:36

A screw acts exactly as an inclined plane. The perpendicular force decomposes and a longitudinal component is generated. This is balanced by an equal reaction of the material: for instance if you try to fasten a screw on a spaceship, as the screw moves forward, the spaceship will move a little backward.


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