Centripetal Force Washing Machine

[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.

 

[Dale]

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

 

[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?

 

[Dale]

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.

 

[A.T.]

But all the textbooks I have read explicitly state that there is no such thing as a centrifugal force.

https://en.wikipedia.org/wiki/Centrifugal_force

 

[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:

 

[A.T.]

But so there is a tangential acceleration, correct?

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

what cause the pendulum to have an angle, or rather what keeps it from pointing straight down?

Draw a force diagram.

 

[Dale]

A force does make them get pushed to the edge..right?

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.

 

[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?

 

[Dale]

the clothes are initially stationary and then they accelerate to the edge.

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

 

[UMath1]

Tangentially to what?

 

[russ_watters]

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.

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.

 

[russ_watters]

Tangentially to what?

To the center of the washer. As the washer starts to spin, the direction of motion (not acceleration) of everything 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).

 

[UMath1]

So how would it apply in the washing machine case?

 

[UMath1]

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

 

[russ_watters]

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.

 

[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?

 

[Dale]

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?

 

[UMath1]

It will spin in a spiral ccw.

 

[Dale]

It will spin in a spiral ccw.

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?

 

[UMath1]

No

 

[Dale]

Exactly. So in the inertial frame the force is never centrifugal. It speeds up (tangential) and it turns left (centripetal) but never right (centrifugal).

 

[A.T.]

In fact, it is just a matter of choice of freference frame and what you choose to make the "action" and what the "reaction".

The frame dependent inertial centrifugal force isn't part of any 3rd Law action-reaction pair.

 

[UMath1]

I think I understand.

But what force is responsible for the tangential acceleration? And what about the video example in post#6?

 

[russ_watters]

I think I understand.

But what force is responsible for the tangential acceleration?

Friction.

And what about the video example in post#6?

What is your question regarding the videos?

 

[UMath1]

In the first video at 2:13, it appears that even in an inertial frame of reference the pendulum is at a slight angle and not normal to ground. Why?

As for the second video, I am not sure what is going on. My understanding is that the tangential acceleration of the key exceeds it centripetal acceleration and thus it moves farther and farther from the center. But what is the force responsible for it? I am not sure how the heavy weight is lifted. Is it because as the centripetal force increases the tension in the string increases causing a net force on the weight?

 

[A.T.]

In the first video at 2:13, it appears that even in an inertial frame of reference the pendulum is at a slight angle and not normal to ground. Why?

Because that's the only way the pendulum can experience a centripetal force.

 

[russ_watters]

In the first video at 2:13, it appears that even in an inertial frame of reference the pendulum is at a slight angle and not normal to ground. Why?

To expand on AT's reply, the tension in the string supplies the centripetal force. Also, the angle looks quite steep to me, not "slight".

As for the second video, I am not sure what is going on. My understanding is that the tangential acceleration of the key exceeds it centripetal acceleration and thus it moves farther and farther from the center. But what is the force responsible for it? I am not sure how the heavy weight is lifted. Is it because as the centripetal force increases the tension in the string increases causing a net force on the weight?

Yes, the tension on the string provides the centripetal force (as well as carrying the weight of both objects).

 

[UMath1]

So the only way the pendulum in video 1 can receive a centripetal force is by being at angle as this allows the tension to have a horizontal component. Then, if I understand correctly, if the merry-go-round was to spin faster the angle would be greater, correct?

For video 2, as the key rotates, the tension provides it with centripetal force. But what causes more string to be pulled upwards? Is it that: when the key rotates the tension in the string increases causing a net force of the heavier weight?

 

[A.T.]

For video 2, as the key rotates, the tension provides it with centripetal force. But what causes more string to be pulled upwards? Is it that: when the key rotates the tension in the string increases causing a net force of the heavier weight?

Yes, there is a Newtons 3rd Law reaction to the centripetal force on the key, which acts on the string. But it's not related to the acceleration of the key, as it acts on a different body. Also note that this reaction to the centripetal force that is different from the frame dependent inertial centrifugal force (see table in the second link):

https://en.wikipedia.org/wiki/Centrifugal_force
https://en.wikipedia.org/wiki/Reactive_centrifugal_force

 

[art sanchez]

The clothes are given a velocity, I assume due to the friction against the walls of the washer. Once they have a velocity (in a straight line) they want to keep that velocity. If no other forces were present the clothes would fly out of the machine in a straight line forever. But, there is a force against the wall of the washer (there's also gravity) which causes an acceleration (change in the direction of velocity). The clothes will want to keep flying in this new direction until they crash into the washer again which in turn changes the angle of velocity, again. The net force is always pointed toward the center of the circle and this is what you call centripetal acceleration.

 

[sophiecentaur]

The clothes are constantly being pulled / pushed into the centre, the water just carries on along a tangent. Hence they are separated.

 

[Let'sthink]

Naming of different forces is creating all the confusion regarding centripetal, centrifugal and so on. Also the term force which occurs in the first two laws of Newton is a different entity than the term action reaction which occur in the third law in place of 'force'. The 'force' that occurs in first two laws represents the net force which may be a single force or a vector combination of many. The 'force' referred in the third law hints at the origin of force that every single force is a result of some interaction. If we understand this or grant it to be true. Then we can solve this naming muddle in a systematic sure way. I will just take two names to illustrate what I am saying:
1. centripetal fore, and
2. gravitational force
The first one just tells you the direction in which the net force is acting, we need to find which interaction or interactions are making that possible.
The second one s[ells out the interaction between Earth and the object in question and focuses on the force acting on the object.

Now these Newton's laws are applicable only in inertial frames. Naturally centripetal force which is experienced only in a non-inertial frame cannot come into picture. In that sense the centrifugal force is called imaginary or pseudo.
For systems in non inertial frames there are two ways of solving the problem. Solve it in inertial frame and then switch over to the non-inertial one. There is another shortcut, wherein we use the Newton's laws with forces of interaction and add a term equal to ma, where a is the opposite to the acceleration of the frame referred to inertial frame. To explain this simply. let as assume that we are in a freely falling lift and in front of us a ball when left also falls freely. . The ball appears at rest to us. If we apply the Newton's law to its motion, then we can conclude that there is no force acting on the ball but we know Earth is pulling both of us. The answer to this dilemma is this that we tell our selves that Newton's laws do not apply in this frame. But if we wish to still make the laws valid for this situation also, we can say that In addition to the force due to Earth there is an upward force mg acting on the object. These two cancel so the object is at rest. This is what you experience as you stand on the floor of a rotating drum with respect to the rotating floor. floor is moving centripetally you are moving tangentially in inertial frame but radially out with respect to the floor. So as long as you are on the floor you can tell yourself that the floor is in no way pushing you out radially. But this thinking gets a jolt as you reach the wall of the drum. Your tendency to go out radially out of the wall causes a reaction by the wall provides you with the required centripetal force necessary to along with the vertical wall with your back touching the wall for convenience. So the so called centrifugal force has caused interaction between the wall and your back to provide the centripetal force required for your motion. In this sense so called centrifugal force, which is not due to interaction is causing an interaction force to be created.