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Re: Centrifugal force



No, actually I know the ball has a net inward force, provided by the string. The reaction force is on the string, not the ball. Without the ball pulling it outward, the string would likely hang more nearly vertical, since its center of mass is closer to the center of the circle. I'd need more time to look at the dynamics of the string to say for sure, but I know, and think I stated below, that the N3 forces are applied on different objects, ball and string.
The centrifugal force on the earth keeps it from falling to its center, not the sun, and results from molecular forces holding the crust on top of the mantle, etc. Similarly there are centrifugal forces on the sun (radiation pressure) that keep it from collapsing under its own weight.
In the rotor, I am pushing outward (centrifugally) on the wall, as could be shown by having me back up to a scale.
skip

-----Original Message-----
From: Joseph Bellina [mailto:jbellina@SAINTMARYS.EDU]
Sent: Monday, November 17, 2003 11:41
To: PHYS-L@lists.nau.edu
Subject: Re: Centrifugal force


Interesting response. Lets first look at the motion of the bob...its
moving in a circle so it is accelerating if you view if from the frame
of reference of the room you are it. N2 implies there needs to be a net
force pointing inwards to account for the inward acceleration, if you
wish the centripetal acceleration. If there is outward force as you
suggest, and if it has the same magnitude as the inward one, by a
missuse of N3 (missuse because N3 do not apply to forces acting a singe
object), then you have contradiction. If the net force is zero, then
there can be no acceleration, but there is. The solution is to
recognize that there is no outward force, only the inward one. N3
cannot be used to imagine such a force, if you did N2 would be violated.

I think you question perhaps comes down to, "if there is no outward
force, then what keeps the bob of the pendulum out there"? Another form
of the question is "If there is not outward force on the earth, why
doesn't it fall into the sun and burn up?" Is this a central question
for you. If it is, we need to address it separately.

So what is the centrifugal force. Think of the fairground ride called
the rotor. You get in, it starts spinning and the floor falls away, but
you stay held against the wall. In the earth frame, the only horizontal
force acting on you is the horizontal component of the normal force by
the wall (the wall may not be completely vertical.) This is true for the
same reasons I described above.
In your accelerating frame you feel an outward force pushing you against
the wall. That is the centrifugal force. It only exists in the
accelerating frame. A similar situation existed in an accelerating car.
When the car is accelerating forward, what force pushes you back in the
seat? You feel pushed back but that is your sensation of the forward force
by the seat on you. If the driver slams on the brakes, what force pushes
you into the windshield, or hopefully into the seat belt...there is none,
you continue to move forward (N1) until the windshield or the seat belt
slows you motion.

You might want to ask yourself if you understand why it is that a bowling
ball keeps moving down the alley even though there is no force acting on
it...or do you think there is a force acting on it which keeps it moving
down the alley. If you think there is such a force than we have more to
chat about.

I hope that helps.

have a nice day

joe


On Mon, 17 Nov 2003, Kilmer, Skip wrote:

Thanks, Joe. Let me see if I understand. Suppose a conical pendulum w=
ith no friction or air drag. The "centripetal force" on the ball is t=
he horizontal component of the string pulling it inward. The "centrif=
ugal force" on the string is opposite to the centripetal force on the=
ball, and acts to pull outward on the end of the string. Ball and st=
ring (c.o.m.) have different accelerations because each has a differe=
nt mass and different other forces acting on it. Is that what you're =
saying?
I thought earlier posts were implying a centrifugal force on the ball=
, which exists only in a frame of reference rotating with it, resulti=
ng from the ball's inertia.
skip=20

-----Original Message-----
=46rom: Joseph Bellina [mailto:jbellina@SAINTMARYS.EDU]
Sent: Monday, November 17, 2003 11:07
To: PHYS-L@lists.nau.edu
Subject: Re: Centrifugal force


A number of folks have responded to this well. I just want to point =
out
that will due respect, your question implies that you may be confusin=
g
issues having to do with the second law, the forces acting on a singl=
e
object, and the third law, equal but opposite forces acting on differ=
ent
objects.
This is a common problem for students, and shows up often through the
use of the words "equal and opposite" that has a very different meani=
ng
in the context of the 2nd law than it does in the context of the thir=
d.

It is a good question to ask ourselves and our students to be sure we
understand the conceptual difference.

cheers,

joe

On
Mon, 17 Nov 2003, Kilmer, Skip wrote:

I've never really understood Physics teachers' distaste for the phr=
as=3D
e, centrifugal force. Doesn't N3 tell us that for every centripetal=
f=3D
orce on an object there is an equal centrifugal force on another ob=
je=3D
ct?
skip=3D20


Joseph J. Bellina, Jr. 574-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556


Joseph J. Bellina, Jr. 574-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556