0
$\begingroup$

Let's say I have a bar attached to the rim of a rotating wheel, sketch, and it's in a vacuum so we can ignore drag.

If the wheel is rotated, does the bar experience a lateral force (perpendicular to the bar)? What if the rotation speeds up impulsively: will the bar experience a lateral force then?

$\endgroup$
3
  • 1
    $\begingroup$ Are you asking about an internal force (stress) in the bar or an external force on the bar? $\endgroup$
    – hazzey
    Oct 23 '15 at 0:54
  • $\begingroup$ External, I think. If you put a force sensor on the bar at the location of the arrow, would it measure anything? $\endgroup$
    – KAE
    Oct 23 '15 at 12:49
  • $\begingroup$ @KAE - What's a "force sensor"? Do you mean a miniature version of bathroom scales? I suspect you want an internal force but don't understand what it means. $\endgroup$
    – AndyT
    Oct 23 '15 at 13:41
4
$\begingroup$

In a vacuum acceleration is all towards the centre of rotation. Acceleration and force are vectors in the same direction, hence with no lateral acceleration there is no lateral force.

In the real world it's not in a vacuum and hence there is an additional force acting on the bar, caused by drag. This drag is lateral to the bar.

An increase/decrease in rotational speed (assuming the impulse for this change of speed comes from a torque at the centre of rotation) would cause an angular acceleration, i.e. the acceleration would not be towards the centre of rotation. As previously mentioned, acceleration and force are vectors in the same direction, hence the angular acceleration (which is lateral to the bar if taken over an infinitely small time period) would result in a lateral force on the bar.

$\endgroup$
5
  • $\begingroup$ In the case of angular acceleration causing a lateral force on the bar, this is an external force on the bar, not an internal force, right? i.e. it would be felt by a mouse sitting on the bar at the arrow location? $\endgroup$
    – KAE
    Oct 23 '15 at 16:20
  • $\begingroup$ @KAE - Yes, a mouse on the bar would feel a force in the case of angular acceleration. $\endgroup$
    – AndyT
    Oct 28 '15 at 17:14
  • $\begingroup$ And this is an external force, not an internal force, right? $\endgroup$
    – KAE
    Oct 28 '15 at 21:20
  • $\begingroup$ @KAE - There is an external force acting on the system somewhere. I don't know where it is. Let's say this force is applied to the centre of the wheel. The wheel then applies an external force to the bar, at the connection between the wheel and the bar. If there is a mouse sitting on the bar, then the bar will apply an external force on the mouse. If there is no mouse, then there is nothing on the bar to "experience" an external force. $\endgroup$
    – AndyT
    Oct 29 '15 at 9:19
  • $\begingroup$ @KAE - To put it another way: force = mass times acceleration. If there is an acceleration in a body, then it must have a force applied to it. This force is an external force. $\endgroup$
    – AndyT
    Oct 29 '15 at 9:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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