I am assuming that an angular velocity vector only has two direction: positive for counterclockwise and negative for clockwise.

Just to make sure that I have the right interpretation.

Newton's 1st law states:

An object remains in a state of uniform rotational motion unless acted upon by a net torque. ("In a straight line" is taken out)

This means that the speed of the angular velocity vector is constant. However, even without an net torque, the direction will change. how?

Edit. I just googled Newton's 1st Law. It only says uniform motion in a straight line. Does it mean the object can travel back and forth along the line?

  • 1
    $\begingroup$ Understand the vector of angular momentum better, here: physics.stackexchange.com/q/219890 . Newton's 1st applies to pure rotation as it does to pure translation. $\endgroup$
    – Gert
    Nov 22, 2015 at 16:14
  • $\begingroup$ @Gert, how to we measure the angle between the force and position vectors? Sin270 will be -1. $\endgroup$ Nov 22, 2015 at 17:41
  • $\begingroup$ I'm not sure what you mean. So $\sin 270^\circ=-1$, so what? $\endgroup$
    – Gert
    Nov 22, 2015 at 17:52
  • $\begingroup$ @Gert, my book did not limit the range of theta to [0, 180]. (Formula: t=rFsintheta) $\endgroup$ Nov 22, 2015 at 17:53
  • $\begingroup$ It doesn't have to: an angle is an angle is an angle, period. $\endgroup$
    – Gert
    Nov 22, 2015 at 18:02

1 Answer 1


By uniform motion in a straight line, the law refers to both its magnitude and direction. You can't go back and forth without a force.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.