I would like to know if centrifugal force is only apparent why all the objects in a washing macchine are pressed against the "wall"? I can experience the effects of the centrifugal force even if I am a still observator?


1 Answer 1


It's not difficult to show that anything moving in a constant speed in a circular path has an acceleration (rate of change of the velocity vector) towards the centre of the circle. The objects in the washing machine drum have just such an acceleration. But according to $\vec{F}=m \vec{a}$ they must experience a so-called centripetal force towards the centre of the circle in order to have this acceleration. The force comes from the walls of the washing-machine drum, pushing inwards on them. In accordance with Newton's third law of motion, the objects exert an equal and opposite 'outward' force on the drum. These 'contact' forces are electromagnetic in origin.

We've been looking at what's going on as 'outside observers', in a so-called inertial frame of reference, in which the drum is turning round. An inertial frame of reference is one in which Newton's laws apply. Indeed we applied the first and second law in the first paragraph (above). But in a frame of reference moving round with the objects in the drum, the objects aren't accelerating and we may imagine that a so-called centrifugal force acts outwards on them, balancing the very real inward force on them from the drum. The centrifugal force is called a fictitious force or a pseudoforce, because it can't be classified as any of the fundamental forces (electromagnetic etc.) and there's no external body that exerts it. Newton's laws don't apply in this accelerating frame of reference.

You may object to talk of imagining a centrifugal force : "But I would feel the centrifugal force acting 'outwards' on me!" I think not. What you would feel are stresses in your body, which arise from different-from-usual forces between adjacent bits of your body. These (essentially electromagnetic) forces and the consequent pattern of stresses are identical whichever frame of reference you're viewing things from. [And the same goes for a simple accelerometer (such as a body on a spring); This measures the strain in the spring, hence the stress.]

So are centrifugal forces real? They're not real to the extent that they can't be classified as any of the fundamental forces and that you can't find an external body which is exerting the centrifugal force on the body which is going in a circle. As I said, in a non-inertial frame of reference Newton's Laws don't apply. So I'd say that in such a frame the concept of force breaks down and, although bits of the concept can be salvaged, it's fairly meaningless to argue whether centrifugal force exists or not. But it doesn't really matter: what does matter is that there is a characteristic pattern of stresses in a body going in a circle, and that's independent of frame of reference.

If you're just beginning a serious study of Physics, it's much better to try and understand the explanation I gave in the first paragraph (that is to understand Physics in an inertial frame of reference where Newton's laws apply) than to concern yourself with non-inertial frames.


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