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Things spinning in circles

  1. Nov 28, 2007 #1
    This is what i dont get for things spinning in circles.

    So let's say you tie a mass to the end of a string, and you start swinging the string around in a circle with a constant speed. In the absence of air resistance, then the net force on the mass would be the centripetal force provided by the tension, which is directed inward of the circle.

    Why then, do objects tend to go outward if the net force is inward?
    For instance, if you used a spring instead of a string, the spring would elongate, but shouldnt the spring compress because the force is directed inward?
  2. jcsd
  3. Nov 28, 2007 #2

    Shooting Star

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    When an object goes round uniformly in a circle, there has to be a centripetal force directed toward the centre. Suppose the object is tied to a spring, as you have said. Then the spring has to exert the centripetal force. The only way a spring can exert any force inward on the object is by stretching.

    You'll come across explanations like the body wants to fly off at a tangent due to inertia, but is not allowed to by the string, and so the net result is that it exerts a force on the string or spring. I think the first picture is more suited for Physics. And I am not going into pseudo forces.
  4. Nov 28, 2007 #3


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    In the case of the spring, the centripetal force is generated by the tension in the spring itself; its physcial tendency to contract - and yes, it is trying to pull the spring towards the centre. It can't because of the angular momentum (the fictional "centrigual force").

    But you can directly observe this inward force: if you suddenly slowed the speed of your rotation, lowering the angular momentum, the spring would indeed contract proportionately until it reached a new equilibrium between the opposing forces.
  5. Dec 3, 2007 #4
    Go back 500 years to Galileo.

    He said that all objects seem to prefer to travel in a straight line unless some force acts upon them.

    And it seems that he was probably right. Newton and Einstein based their famous theories on that simple idea. It's a very useful idea to help you think straight in mechanics/physics.

    So the mass on the end of the string is merely trying to move in a straight line (always is).
    The rest is pretty obvious. Explicitly: to change from straight line motion to anything else requires a force. If the object is moving in a perfect circle, that force is centripetal (towards the center). For perfect circles, a constant force must be applied at a right angle to the velocity (ie at precisely 90' to the direction of motion).

    If that force is removed, the object moves off in its prefered state: - a perfect straight line. Simple enough.

    answer: They don't. They're just trying to go off in a perfect straight line, just like Galileo said they would. ;-)
  6. Dec 4, 2007 #5
    Because the forces have to balance.

    Thought experiment time:

    A) No air resistance
    B) You are a free floating body in space, not say a kid on the planet with a chestnut on a string.

    1) Cut the string. What happens? Nothing. The object orbits about you.

    2) Same experiment. Stop spinning. What happens? The mass maintains it's momentum and orbits you.

    Assume B is wrong and you actually are[/ a boy on the planet with a chestnut or reasonable facsimile on a string

    1) Cut the string. What happens? The chestnut falls at 9.8 meters per second squared and apparently departs at a tangent to it's release point.

    So... Wha happen???

    The object (the chestnut) stopped orbiting you as it was constrained by the artificial string and attempted to orbit the body you are attached to (The Earth). Unfortunately it's velocity was lacking to place it in orbit. (Note to tester, use a bigger string..) as a result, the chestnut (or reasonable facsimile) entered a ballistic trajectory where the escape vector was negative, and while still leaving the string at right angles to the release point, it fell at 9.8 meters per second squared to the earth.

    Note: If the chestnut was say about the weight of a .302 round and the string was moving really fast, say about 1400 m/s at the tip and you angled up a bit it could go quite a distance. Think shooting guns in the air in latin american countries to clebrate cinquo de mayo for instance as an example.
  7. Dec 4, 2007 #6

    Doc Al

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    The force on the object is inward, but the force on the string/spring is outward. The string/spring is under tension. And the only way a spring can exert a force is by being stretched--the more inward force you need, the more the spring must stretch.
  8. Dec 4, 2007 #7


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    If you are saying what I think you are, it is completely wrong. Really, badly wrong.

    If you were in space twirling a chestnut on a string, the string would be tight, and if cut the chestnut would fly away tangentially - just as it would on earth.

    What would hold the chestnut in orbit around you if the string was cut ?

    If that's not what you mean, please disregard this.
  9. Dec 4, 2007 #8

    this does not seem correct. What about our solar system, if the sun disappeared would are planet continue to have an elliptical orbit or fly off tangentially. Newton would argue that our planet would fly off instantaneously, Einstein would say it would take 8 minutes or is it 4, :). Maybe I misunderstood your thought experiment please elaborate.
  10. Dec 4, 2007 #9
    This question has been discussed a lot among my friends. I think you all explain quite clearly already. I just add a little bit: why not use the Third Newtonian Law?.

    It simply is:
    'For every action, there is an equal and opposite reaction'.

    So, the string (or spring) exerts a force on the mass and keeps it in circle. In turn, the mass exerts the exact force on the string but in opposite direction.
  11. Dec 4, 2007 #10
    Because I think if you did that, you would be analysing the problem from the point of view of somebody moving WITH the rotating object (pretending you hadn't noticed the circular motion at all). It would be an accelerated frame of reference which good physicists & engineers try to avoid using.

    Initially it would look good. You would have reduced the problem to a simple statics problem where all the forces balance and are in equilibrium.

    But when the string breaks, the object would appear to move away from you in a peculiar spiraling motion. - How would you explain that? It could be done, but all your maths would suddenly become rather complicated...

    It's like when you watch somebody fall off a carousel (merry-go-round), or maybe you throw a stone outwards whilst on a merry-go-round.
    It's partly the reason your get disorientated immediately when you step off (or fall off) a rotating object like a carousel. Because whilst you were on it, you'd got used to treating the carousel as your reference frame...

    I'm not certain if I'm 100% right here to be honest, but that's my 2 cents anyway.
    The first answer here was the best one. Actually he said it all in a couple of sentences. We're all waffling about nothing much really ;-)
  12. Dec 4, 2007 #11


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    I think not. If you were facing the object it would appear to move off in a straight line.
    If you're off the carousel it appears to move in a tangential straight line.
  13. Dec 4, 2007 #12
    No matter which way you're facing on your rotating reference frame, the object will never appear to move in a straight line. It only looks straight to somebody in either straight line motion (running past) or somebody standing still by the fence.
    - exactly.
    And if you're on it and treating it as your reference frame, then it simply doesn't appear to move in a straight line at all.
  14. Dec 4, 2007 #13


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    You are right in most particulars. I chose a rather special frame, at the center of the rotating frame, which rotates so it always faces the object - like keeping the object in the middle of a camera view-finder.

    Anywhere away from the center this is not possible. Presumably what you see then is like the apparent retrograde motions of the planets ?
  15. Dec 4, 2007 #14
    Sure enough, somebody not rotating at the centre isn't really 'on' the rotating frame at all.. He may as well be standing over by the fence watching the show from there.

    Actually I thought for a moment I was wrong, I'm no expert, I just like thinking about physics. Completed my physics degree 20 yrs ago. Never really used it since...;-)
  16. Dec 4, 2007 #15


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    Note that the object will still not travel in a straight line even from this reference frame.

    Once the string is cut, the rotating FoR will only keep it in sight through another 90 degrees before the object continues on its course.

    Either that or, once the string is cut the FoR must come to a stop within a mere 90 degrees rotation, else its "window" will carry around the far side of its revolution, whereas the object will not.
  17. Dec 5, 2007 #16


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    Yes, it will, I'm keeping the object in the middle of the viewfinder.

    Anyhow, you guys have beaten me up enough, I'm gone.
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