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student34
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Homework Statement
what about the reaction force to a ball swinging in circles on a string or a rollercoaster doing a loop.
Simon Bridge said:You need an unbalanced force to get an acceleration.
In the ball-on-a-string example, the ball pulls on the string only in the reference frame of the ball ... i.e. non-inertial. Relative to the person holding the string - they are pulling on the string, and, therefore, the ball. The pull is an unbalanced force pointing to the center, causing the acceleration.
The centrifugal "force" is an effect that comes from an accelerating reference frame.
We call these "pseudo-forces".
An easier example of a pseudoforce is when you are in an enclosed box (inside a cargo container maybe) which has a light-fitting (so you can see) suspended from the ceiling. You notice that the light hangs at an angle to the ceiling ... the mysterious force holding it at the angle is a pseudoforce, what's happening is that the box you are in is accelerating.
Newton's third law does not balance forces.student34 said:But my textbook also says that Newton's 3rd law implies that object A's force on object B will be equal and opposite to object B's force on object A. So I still don't understand how there can be an unbalanced force if Newton's 3rd law holds.
student34 said:But my textbook also says that Newton's 3rd law implies that object A's force on object B will be equal and opposite to object B's force on object A. So I still don't understand how there can be an unbalanced force if Newton's 3rd law holds.
ehild said:The swinging ball feels the pull of the string, the tension in the string. The ball pulls the string, so the string feels the reaction force of its pull.
ehild
Simon Bridge said:Newton's third law does not balance forces.
eg. gravity pulls down on you as you sit in your chair.
The reaction force to this acts at the center of the Earth and pulls the Earth upwards towards you.
There is also a force from the chair that stops you from falling. You press equally hard on the chair.
Remove the chair and the gravity force is said to be "unbalanced" - you accelerate towards the Earth and the Earth accelerates towards you.
To have an acceleration, the forces must be unbalanced in this way.
In circular motion, there must be an acceleration since the velocity changes direction.
This follows from the definitions of velocity and acceleration as vectors.
The instantaneous direction of the change in velocity is towards the center.
Hence there is an unbalanced force acting towards the center.
Assuming that centrifugal force is a real force the way Newton defined them:
For the ball on the string, the tension in the string, the weight of the ball, and the centrifugal force, sum to zero. This means the ball is not accelerating. But we know it is accelerating because we can see its velocity is changing. This is a contradiction - so we discard the assumption.
The effect is real - it is just unhelpful for you to think of it as a force.
Yes, there is a 3rd Law reaction force (acting on object B) to the centripetal force (acting on object A). This reaction force is sometimes called "centrifugal", because it acts outwards (depending on how you define your objects). This force is a "real" interaction force and exists in every reference frame:student34 said:But my textbook also says that Newton's 3rd law implies that object A's force on object B will be equal and opposite to object B's force on object A. So I still don't understand how there can be an unbalanced force if Newton's 3rd law holds.
yes, but this force is acting on the string.student34 said:Isn't the ball's pull a force in the opposite direction from towards the center?
A.T. said:Yes, there is a 3rd Law reaction force (acting on object B) to the centripetal force (acting on object A). This reaction force is sometimes called "centrifugal", because it acts outwards (depending on how you define your objects). This force is a "real" interaction force and exists in every reference frame:
http://en.wikipedia.org/wiki/Reactive_centrifugal_force
There is a "real" force called centrifugal, and a "fictitious" force called centrifugal. See Wiki links above, and this one:student34 said:So there really is a centrifugal force.
student34 said:There is something else odd about all of this. My grade 12 physics notes and textbook talks a lot about centripetal force. But my university physics textbook only ever mentions a(rad), never once mentions "centripetal force".
Well said.AlephZero said:inventing logic-chopping terminology like "reactive centrifugal force" doesn't add any value IMO.
Well said again. That replicates my experience. My take is that this concept has a small but rather vociferous following. Wikipedia is an avenue that let's those voices be heard.And in 30 years of reading peer-reviewed papers on the dynamics of rotating machinery, I've never seen it in print, outside of Wikipedia.
http://www.google.de/search?tbm=bks&q="centrifugal+reaction"AlephZero said:And in 30 years of reading peer-reviewed papers on the dynamics of rotating machinery, I've never seen it in print, outside of Wikipedia.
The problem seems that people conflate the two distinct forces and get confused. Wikipedia clears up the confusion by pointing out the two separate meanings.D H said:Wikipedia is an avenue that let's those voices be heard.
D H said:Well said.
Well said again. That replicates my experience. My take is that this concept has a small but rather vociferous following. Wikipedia is an avenue that let's those voices be heard.
Depends which centrifugal force, which object and in which reference frame. But arguments about "reasons" get rather philosophical. The net force (and initial velocity) determine the path.student34 said:So by definition, centrifugal force exists, but it is not the reason for the object's path it takes.
student34 said:But my textbook also says that Newton's 3rd law implies that object A's force on object B will be equal and opposite to object B's force on object A. So I still don't understand how there can be an unbalanced force if Newton's 3rd law holds.
namanjain said:Newtons laws exist in inertial frame of reference while pseudo force act due to non inertial frames...
on respect to question drawing FBD respect to car turning anticlockwise
-----→centrifugal force
a pseudo force wrt car of a passenger inside
A.T. said:Depends which centrifugal force, which object and in which reference frame. But arguments about "reasons" get rather philosophical. The net force (and initial velocity) determine the path.
Simon Bridge said:I think I see where the confusion lies - there are two uses of the word "centrifugal":
Any radially outward pointing force may be called "centrifugal" because that is what the word means.
However, what you'd commonly think of as "the centrifugal force", the sensation of something pulling you towards the outside of a curve, is an illusion - as your own reference emphasizes:
For instance, if you are riding in an automobile rounding a curve at high speed, you have to hold on to the edge of the seat to keep from sliding outward, and this gives you the sensation that something is pulling you to the outside of the curve, as though your weight had acquired an extra, centrifugal component. However, you are suffering from an illusion. There is actually no such centrifugal force pulling you outward ...
(My emphasis).
From: Dynamics of Uniform Circular Motion ch6 MIT "Pivot" Text Book.
This is in agreement with your textbook - which is assuming common understandings.
A centripetal force is a force that acts towards the center of a circular motion, keeping an object in that motion. It is necessary for an object to maintain a circular path.
The textbook most likely states this because a centrifugal force is not a real physical force. It is a perceived or apparent force that occurs due to the inertia of an object in circular motion. It is actually just an effect of the centripetal force.
The concept of a centrifugal force can be understood by considering Newton's First Law of Motion, which states that an object in motion will continue in a straight line unless acted upon by an external force. In the case of circular motion, the object wants to continue in a straight line, but the centripetal force acts towards the center, causing the object to move in a circular path. The inertia of the object causes it to resist this change in direction, creating the illusion of a centrifugal force.
The term "centrifugal force" is still commonly used because it is a convenient and practical way to describe the outward force experienced by objects in circular motion. It is also a concept that is familiar and easily understood by many, even though it may not be scientifically accurate.
Yes, there are many real-life applications of the concept of centripetal and centrifugal forces. Some examples include the motion of planets around the sun, the rotation of tires on a car, and the spinning of a washing machine. Understanding these forces is essential in fields such as engineering and physics, as they are crucial in designing and analyzing circular motion systems.