At a recent family gathering, the subject of centripetal force came up. One family member insisted that centrifugal force does not exist, and that centripetal force is the only force (in this context). This member insisted that when people generally refer to centripetal force, they are in fact wrong and that they should really be saying centripetal force instead. Two other opposing family members insisted that centrifugal force is the force imposed AWAY from the center, and that it opposes centripetal force that imposes a force TOWARD the center. Kind of a “you cant have one without the other” type of thing.
The centripetal force is there every time something is rotating, simply because it wouldn’t be rotating without it - the centripetal force is directed at the center of the circle, keeping the rotating object on its orbit instead of flying away. The centrifugal force is directed outwards, away from the circle; it results from the object’s inertia, because it resists the change of its movement. At any point on the circle, the object is trying to fly on straight forward, but it’s getting kept on its orbit by the centripetal force. As a result, inertia is pulling the object outward, tangentially to the orbit of the object. If it’s a continous stable rotation, centripetal and centrifugal forces are of equal value, but the first is pulling inwards, the other one outwards.
The direction of the centrifugal force is not tangentially outward, sorry. The object is trying to fly on straight forward, tangentially to the orbit; the centrifugal force, a result of inertia, is directed just straight out of the circle (at a 90° angle to the direction the object is moving to at any given moment).
Schnitte is incorrect, if the centripetal and centrifugal forces were equal, the object would not accelerate towards the center, and would go straight instead.
Centripetal force is the only ‘real’ force acting on an orbiting object. Imagine you are sitting in the passenger seat of a car, and it turns sharply left. You will be ‘flung’ towards the door to your right, and pressed against it during the turn. People say that is centrifugal force, but there is NO force pushing you right, NO force pressing you against the door, you are trying to go straight, and the car door moves into your path, pushing you left instead.
Since you’re being pushed against the door, you ‘think’ there must be something pushing you that way. See, even I said “pushed against the door” when there is nothing actually pushing you that way, you were already going that way.
One very important thing to remember about physics, if the forces zero out, no acceleration takes place. If there is no acceleration, things either sit still or move in a straight line however fast they were moving initially. This never, ever, changes, if you have a situation that seems to violate this, you’ve messed something up.
I’d have to concur with Cheesesteak on this. I’m not sure where the idea of a centrifugal force first came from, but I offer as one point of data the fact that my college physics text (engineering-type physics) there is no mention of centrifugal force in the index, only centripetal force. It does, however, offer the following:
“…consider a car traveling along a highway at a high speed and approaching a curved exit ramp. As the car takes the sharp left turn onto the ramp, a person sitting in the passenger seat slides to the right across the seat and hits the door. At that point, the force of the door keeps him from being ejected from the car. What causes the passenger to move toward the door? A popular, but improper, explanation is that some mysterious force pushes him outward. This is often called the “centrifugal” force. The passenger invents this ficticious force in order to explain what is going on in his accelerated frame of reference.”
So I guess you could say that centrifugal force is related to inertia as Schnitte said, but it’s really an artificial construct to explain the effects of inertia from the perspective of someone who is a part of the inertial system they are observing (i.e. the car passenger described above)
I agree El Marko. My college physics text doesnt mention centrifugal either…only centripetal.
However, in the “real-life” world, if you had a chainsaw that was malfunctioning… So you go to the parts store and tell them you need a CENTRIPITAL clutch??? Society has adopted centrifugal as the norm. But why? And why is this wrong?
Main Entry: 1cen·tri·fuge
Pronunciation: 'sen-tr&-"fyüj
Function: noun
Etymology: French, from centrifuge centrifugal, from New Latin centrifugus
Date: 1887
: a machine using centrifugal force for separating substances of different densities, for removing moisture, or for simulating gravitational effects
Why does centripetal have no place in this definition or why isnt this machine called some derivitive of centripetal?
This is the essence of the situation. I think if a physics text says that centrifugal force is “improper” then it’s not an advanced enough textbook. It’s proper to use it as long as you’re clear that you’re dealing in a rotating reference frame and can handle that fact.
What about the coriolis force? Does the textbook mention that? It’s also one of those “fictitious” forces that only exists in a rotating reference frame. My college texts talked about it.
Thus in one respect, as CurtC says, the centrifugal force is a real force but since it still doesn’t have a source it remains to some extent a pseudo force.
This is easy. The centrifugal force is fictional in an intertial (newtonian) reference frame, it is real in a rotational reference frame.
We describe forces in order to explain something that is going on, if we are consistent in doing so then all types of reference frames are equally valid and in some contexts you could legitimatelly refer to centrifugal force.
Everyone here should complain about their professors; my high school professor made this issue fairly clear (my college professor didn’t really, but hey)
There was some discussion about this last week (in a hijack of another thread) here.
My $0.02:
First of all, centripetal force is the only proper rotational force to consider if you are looking at a problem from an inertial (non-accelerating frame). In an inertial (Newtonian) reference frame, centrifugal force does NOT exist.
Centrifugal force is only added if you are looking at a problem from a noninertial (accelerating) reference frame.
Centrifugal force is equal in magnitude and opposite in direction to centripetal force. Since centripetal force points inward, centrifugal force points directly outward.
Cheesesteak, centrifugal force and centripetal force are indeed equal in magnitude. In a rotating reference frame, they do indeed cancel out–that is the whole point. Remember, centrifugal force can only be considered when you are in a noninertial (accelerating) reference frame, such as a rotating reference frame.
I can’t see what the big fuss is about. It is fairly simple and it seems to me the discussion here is more about semantics than about physics.
Take any mass and apply a force to it and it will accelerate with an acceleration a = F/m. You can say that a mass being accelerated creates an opposite force of magnitude F = a * m which is pretty much saying the same thing. In the case of rotary movement the centripetal force is the active (“real” if you want) and the centrifugal force is the reactive force.
Friction is similar in that it creates forces which do not exist on their own but exist as a consequences of other things. Move something to the right and you feel a force to the left. And yet, move it to the left and you feel the force pulling to the right. Leave the object in place and there is no force. Centrifugal force is similar to the forces created by friction. They exist as a consequence of other things and cannot exist on their own but they are definitely real.
The discussion here is indeed partly about semantics. However, Sailor, your post also shows why the concept of centrifugal force is so (needlessly) confusing.
Reactive forces do exist in an inertial reference frame. They are as real as “active” forces. Centrifugal force (acting on a body undergoing rotational motion) does NOT exist in an inertial reference frame. The problem is that some people refer to centripetal force’s reactive force as “centrifugal force.” The difference, however, is that reactive forces (in the sense of Newton’s third law) do not act on the same body as the “active” force.
For example if I swing a ball around in a horizontal circle* at constant speed, there is a centripetal force on the ball that causes it to constantly accelerate with uniform circular motion. The reactive force in the case does not act on the ball. It acts on my hand. It is equal in magnitude and opposite in direction to the centripetal force acting on the ball. This reactive force is a real force that is present even in an inertial reference frame. While this force is equal in magnitude and in the same direction as the centrifugal force that may be considered to be acting on the ball, it is really a different force, since this reactive force acts on my hand, and centrifugal force acts on the ball (in a rotational reference frame).
No force exists on its own. A consequence of Newton’s Third Law is that all forces come in pairs: My hand exerts a force on the wall; the wall exerts a force back on me.
Centrifugal force is different. Both centripetal force and centrifugal force are considered to act on the same body.
This website has a good explanation of the matter, using an example of satellite motion for this first paragraph quoted here:
This last sentence, as with Sailor’s post above, demonstrates how confused the term “centrifugal force” is.
My take on the whole matter is to never use the term.
[sub]*For simplicity’s sake, consider the swing to either take place on a horizontal frictionless surface, or in a weightless environment.[/sub]
