# Force applied on suspending/falling mass

Hi,

May I ask, if there is a sufficiently long rod falling vertically downwards due to gravity, and then at an instance, a horizontal force is applied at one end of the rod, will this force create a torque on the rod? If so, where is the axis of rotation?

Second question, if a body, not hinged, is experiencing rotation, e.g. a flipping car in "mid air" in a collision, does the axis of rotation ALWAYS go through its center of gravity. If not, how do we find out which axis?.

Third question, say, in the flipping car example, is the "Net Torque" about any axis independant of any other axis? Is there a method to calculate the "net rotation about net axis" so that we can integrate over time to get the resultant orientation? Similar to integrating velocity vector to get position vector kind of maths?

Sorry, that's aquite abit. Hope I can get some help. Thanks for helping.

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tiny-tim
Homework Helper
Hi jakesee! Yes, if there's a torque (a moment) about the centre of mass, then the body will rotate.

The position of the axis of rotation depends on the exact figures (it won't usually be the centre of mass).

The total angle of rotation doesn't depend on the axis. You can get the dynamics from torque = moment of inertia times angular acceleration, just like the linear F = ma.

See http://en.wikipedia.org/wiki/Instant_centre_of_rotation" [Broken] for some details. Last edited by a moderator:
Thanks for the replies, I'll be reading the suggested topics a while before coming back. thanks thanks. =)