1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

COnstant centripetal force to move in a circle?

  1. Mar 28, 2012 #1
    An interesting thought just struck me and I wanted to confirm if it is correct.
    Do all objects (of fixed mass) need a particular magnitude of force to keep them moving in a circle,
    e.g: a ball will ALWAYS need a force of 10N to keep it moving in a circle
    If the speed increases then the radius must ALSO increase to accomodate for the change in speed so as to ensure the centripetal force required is CONSTANT?

    is this idea correct?
     
  2. jcsd
  3. Mar 28, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    No, it's not correct.
     
  4. Mar 28, 2012 #3
    If you had a way of generating a constant centripetal force, yes. That's how satallites get to higher orbits. (I think).
     
  5. Mar 28, 2012 #4
    It would seem sensible though.
    Imagine a CD spinning. the dust particles collect around the centre as all the dust particles are of similar/identical mass and thus ALL require the same force to keep them moving in a circle.

    If the CD span faster, I presume that they would move further out on the CD as the force provided there is sufficent to keep them moving in a circle as the centripetal force required is matched?
     
  6. Mar 28, 2012 #5
    Do you imply that the centripetal force in the higher orbit is the same as in the lower one?
     
  7. Mar 28, 2012 #6
    well in the lower one, surely the speed is less too!! so yes!! (and it depends on mass of the object)
     
  8. Mar 28, 2012 #7
    Going from lower orbit to higher orbit is not so simple so the discussion about speeds depends on what kind of orbits you have and what do you call "speed" in case you have elliptical orbits. For stable circular orbits however the speed is higher in the lower orbit.
    The centripetal force ("provided" by gravity) is lower for the higher orbit (see Newton's law of gravity).
     
  9. Mar 28, 2012 #8

    Doc Al

    User Avatar

    Staff: Mentor

    To expand on my previous answer: Twirl a ball at the end of a string in a circle. What stops you from twirling it as fast as you want? The tension in the string will just increase, since the required force is greater. (Until it breaks of course.)

    You're statement that all objects of fixed mass need the same centripetal force is clearly wrong. It depends on how fast they are moving and at what radius. (Sure, you might conceive of situations where the force remains constant and the radius changes just right to keep it moving in a circle, but that's not true in general.)
     
  10. Mar 30, 2012 #9
    If the difference in height isn't too big, yes. But I see your point.
     
  11. Mar 30, 2012 #10
    another example similar to Doc Al's is imagine a charged particle with some velocity. Then imagine a magnetic field is turned on, and its direction is perpendicular to the particle's velocity.

    The velocity of the particle will remain constant, and it will go in a circle. If the strength of the magnetic field is increased, then the force on the particle will increase, and the radius of the circle traveled by the particle will decrease.
     
  12. Apr 13, 2012 #11
    Hi guys, i am having a nightmare with the l3 btec in mechanical engineering and wondered if anyone could help.

    I need to find out how to calculate the minimum speed required for an object to travel in a vertical circle of 1.5m,

    The 1.5m is that the radius and if so does this look right?

    centripetal force = weight
    mv2/r = mg
    thus v2 = rg
    v2 = 1.5 * 9.81
    v = 3.836 ms-1
     
  13. Apr 13, 2012 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I know that some people just HATE the idea of using or accepting Maths in an explanation but one simple Maths expression says it all.
    The (centripetal) force needed to keep a mass m (kg) on a circular path of radius r (m) at as speed of v (m/s) is

    F= mv2/r (N)

    That shows you that, if you want to increase the speed, the force needs to increase but, if you want to increase the radius, the force gets less.

    If you are discussing Orbits, then the gravitational force will decrease as the radius increases so the sums get a bit more complicated and you can't just assume an unstretchable 'piece of string' is keeping the mass on its path.
     
  14. Apr 13, 2012 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You are right that the weight of the mass is enough to keep it on track when at the top of the circle but you haven't actually said why.
    I think you probably mean the speed at the top? Looks right to me.
    Of course, the speed -on a string, say - wouldn't be constant in this model. (See title of thread)
     
  15. Apr 13, 2012 #14
    Ok thanks for your help. appreciate it
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook