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Please could someone provide an explanation for this. I thought that increased velocity would cause greater centripetal force (which, I think, acts towards the centre of curvature) so would the springs not become more compressed with greater speed and hence the brake pads would become more distanced from the collar?

How does velocity of a spinning object affect an attached spring?

All help much appreciated.

enter image description here

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  • $\begingroup$ Short, very oversimplified answer: the pads have more mass than the springs, and are therefore more greatly affected by the centripetal force. $\endgroup$ Sep 4, 2018 at 20:11
  • $\begingroup$ Anyway, I think your observations about centripetal force might be inaccurate. As the speed of the shaft increases, the brake pads will tend to move away from the shaft, not towards it. $\endgroup$ Sep 4, 2018 at 20:14
  • $\begingroup$ Posted here thestudentroom.co.uk/showthread.php?t=2648793 and then reposted with the mark scheme given here thestudentroom.co.uk/showthread.php?t=2648793 $\endgroup$
    – Farcher
    Sep 5, 2018 at 5:21

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I thought that increased velocity would cause greater centripetal force

Increased (rotational) velocity requires greater centripetal force to maintain circular motion. It does not on its own create the force.

In your diagram, if we ignore gravity the only radial forces on the retracted pads are from the springs.

You can calculate the required centripetal force needed for circular motion given a particular distance and a particular rotation speed. This force increases as rotation goes up.

What happens when the rotational speed increases and the compressed spring can no longer can provide sufficient centripetal force?

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I thought that increased velocity would cause greater centripetal force (which, I think, acts towards the centre of curvature) so would the springs not become more compressed with greater speed and hence the brake pads would become more distanced from the collar?

Your understanding here regarding the centripetal force and the compression of the springs at high angular velocity is correct. But, when springs compress, the pads will move out, i.e., closer to the collar as should be expected for the brakes to work.

If you look carefully at the drawing, you'll see that each spring sits inside a cylindrical well inside a pad: one end of the spring is pressed against the bottom of the well, the other against a head of a bolt.

As the angular velocity changes, the pad moves, radially, in and out, along the length of the bolt. Specifically, when the angular velocity increases, the pad moves out, towards the collar, and compresses the spring against the head of the bolt, leading to the increased centripetal force on the pad.

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