Centrifugal Homepage

Question: Is the reason that planets don't fall into the sun because of centrifugal force?

(Posted by: Inquisitive Curiousity on 2009-11-23 12:33:03)

My physics teacher asked the class "Why is it that our planet doesn't just fall into the sun? " He said that it was because of centrifugal force, which balances out the gravitational force from the sun. I had no clue what he was talking about, so I asked if this centrifugal force was tangent to the planet's motion, or if it directly opposes the gravitational force from the sun. He said that it acts opposite to the gravitational force from the sun, which is why the planet is able to move around the sun without falling into it. Short version: centrifugal force balances out the gravitational force from the sun. Is this correct? It doesn't really make sense to me, because if the two forces balance out, wouldn't the planet then be moving in a straight line from Newton's First Law? He also said that the equation F = mv²/ r refers to the centrifugal force, but I remember it as referring to the centripetal force. Thanks for your answers.

Posted by: squareyes on 2009-11-23, 12:41:39

Your teacher is incorrect. Centrifugal force is not an actual force. When objects move in a circle they have a centripetal force which acts towards the centre so that it remains in circular motion. Also, you are correct in saying that the F = mv^2/ r does refer to centripetal force.

Posted by: Randy P on 2009-11-23, 12:50:02

That's not a correct description. There is no balance of forces. The circular motion is due to an imbalance of forces. Start with a planet moving with some velocity past a sun. The force of the sun deflects its path, so now it has a velocity in a new direction as it moves forward. But the sun deflects its path again, so as it moves forward some more, it changes direction again. The path (ellipse or circle) that a planet follows is the combination of the tangential velocity and the deflecting force that causes that velocity to curve. The basic issue is that a force doesn't automatically cause things to fall straight inward toward that force, it just deflects them. If I throw a ball with some horizontal velocity, the path curves toward earth. But it doesn't turn a right angle and drop straight down toward earth vertically.

Posted by: Hobo McCamera on 2009-11-23, 12:51:48

That is the worst explanation for planetary orbit that I've ever heard. Centrifugal force is actually what's called a pseudoforce, which is a force that appears if you make observations from a non-inertial frame of reference, such as the Earth (which is moving in a circle). If you don't understand what that means, that's cool. I will give you another explanation. Technically, I believe what your teacher was saying is true, but as I said, it's a terrible way to explain that. The Earth is trying to move in a straight line. What I mean is that at any given moment, the Earth's velocity vector is tangent to its orbit. You're right about what that equation is, it's the centripetal force. The force of gravity acts as the centripetal force, which keeps Earth from flying off on a tangent and keeps it moving in a circle. So the reason that it doesn't fly away is because of its velocity. If Earth stopped moving for a moment, it would, indeed, fall into the sun.

Posted by: J A on 2009-11-23, 12:52:02

It is centripetal force, not centrifugal. Technically, there is really no such thing as centrifugal force which is the force of an object moving outward. Centripetal force is correct (and it is inertia which would be the force to cause an object to move outward, like the Earth, if the Sun were to disappear).

Posted by: Fluke on 2009-11-23, 13:02:58

Your teacher was incorrect, but it is a very common mistake. At least he gave you the right formula to work with, so he gets points for that. technically speaking, in the most literal sense, centrifiugal force is just centripetal force applied at a right angle to the apparent "moment of motion " of the object. Since the motion is continuous, the "moment " never really exists, but in a mathematical sense if you could "freeze " everything and study it closely (eliminating Heisenbergs uncertainty) you'd see the forces only balance if the implied force is centrifugal. In the real, physical, non-mathematical world, yeah, its "applied centripetal force " Hobo - isnt the Earth's velocity vector "pependicular " to its orbit?

Posted by: Ethan on 2009-11-23, 13:19:41

Nope. You remember correctly. Your teacher is invoking an unnecessary force. Curved motion implies an acceleration orthogonal to the instantaneous velocity vector...that's your centripetal force (or centripetal acceleration)...that acceleration *is* the Sun's gravity for the case of Earth's orbit. In any case, I suppose that treating gravity as an acceleration is unnecessary too when viewing things in four dimensions...just a matter of geometry. People commonly imagine curved trajectories in 3-space as inertial motion, though. Quite regularly in meteorology, for example. We speak of the Coriolis force as a real thing even though it is 'really' an artifact of viewing air parcels in an accelerated reference frame.