# Centripetal Force Washing Machine

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1. Aug 19, 2015

### UMath1

I know that the centripetal force is the resultant force which points to the center. So in the case of a washing machine, what force pushes the clothes and water to the outer edge? There is not centrifugal force, but then how come all the clothes appear to get pushed to the circumference of the washing machine.

I saw something similar in a science museum. There was a rectangular glass prism with water inside. When it rotated about its center, all the water rushed to the edge.

I asked my science teacher and he began talking about the centrifugal force. But all the textbooks I have read explicitly state that there is no such thing as a centrifugal force.

2. Aug 19, 2015

### Staff: Mentor

If you are examining the motion in an inertial frame then none of the clothes are ever pushed out at any time.

3. Aug 19, 2015

### UMath1

I mean initally before starting the machine the clothes are evenly distributed. Some are in the middle, some are on the edges. Once the machine starts all the clothes appear to rush to the edge. What causes them to do that?

4. Aug 19, 2015

### Staff: Mentor

The bottom of the washer accelerates them tangentially. The sides of the washer accelerate them centripetally. There is no centrifugal acceleration.

What you are assuming is centrifugal motion is not. It is tangential motion.

5. Aug 19, 2015

### A.T.

6. Aug 19, 2015

### UMath1

But so there is a tangential acceleration, correct? A force does make them get pushed to the edge..right? Once at the edge however, the only force is the centripetal pulling force, right?

Also, in this video() at 2:13, what cause the pendulum to have an angle, or rather what keeps it from pointing straight down?

And what causes the ball to swing outwards in this video:

7. Aug 19, 2015

### A.T.

Even movement at constant speed along a tangential line would bring you to the edge.

Draw a force diagram.

8. Aug 19, 2015

### Staff: Mentor

Not in an inertial reference frame, no.

Remember, a force causes acceleration, so it is not enough to see that the distance between the clothes and the rim decreases. You must look to see what object is accelerating in which direction.

There is simply no object in this scenario which accelerates centrifugally in an inertial frame. Nothing gets pushed out.

9. Aug 19, 2015

### UMath1

But when I look at the washing machine from the outside, the clothes are initially stationary and then they accelerate to the edge. So there must be some force?

And what about the video examples in post #6?

10. Aug 19, 2015

### Staff: Mentor

No. They never accelerate to the edge. The accelerate tangentially and centripetally only.

11. Aug 19, 2015

### UMath1

Tangentially to what?

12. Aug 19, 2015

### Staff: Mentor

That isn't true. This is a result of overteaching the idea that the centrifugal force is "ficticious". In fact, it is just a matter of choice of freference frame and what you choose to make the "action" and what the "reaction". It is called "ficticious" because it appears to arise from nowhere when viewed in a rotating frame, but it is quite real.

13. Aug 19, 2015

### Staff: Mentor

To the center of the washer. As the washer starts to spin, the direction of motion (not acceleration) of everthing is tangential to the center. So as the clothes start to move, they try to follow a straight line, which causes them to slam into the drum of the washer. (It is also opposed by friction, so it will really be more of a spiral).

14. Aug 19, 2015

### UMath1

So how would it apply in the washing machine case?

15. Aug 19, 2015

### UMath1

So the tangiential acceleration appears to be centrifugal but isn't?

16. Aug 19, 2015

### Staff: Mentor

I feel like I (and others) just answered those questions. Can you be more specific about what is confusing you? Perhaps quote the exact passage?

Maybe the issue is mixing and matching frames. Maybe pick one and stick with it. People often prefer the inertial (non rotating) frame.

Last edited: Aug 19, 2015
17. Aug 19, 2015

### UMath1

So if we stick with the inertial frame, when the machine turns on the clothes receive an acceleration tangential to the center pushing them to the edge where they experience centripetal acceleration, correct?

18. Aug 19, 2015

### Staff: Mentor

Suppose that the washer empty except for a single small sock which is initially at rest on the bottom of the washer. Now, the washer begins to spin CCW.

Can you describe the path of the sock in the inertial frame?

19. Aug 19, 2015

### UMath1

It will spin in a spiral ccw.

20. Aug 19, 2015

### Staff: Mentor

So if we think of CCW as the sock's path turning to the left, will the sock's path ever turn to the right in this scenario?