Undergrad Why Does a Glass Move Outward When Opening the Fridge Door?

[user079622]

If centrifugal force dont exist why glass of water moves outward/toward me when I open fridge door?
Glass of water moves out in relation to door and in relation to earth, so what force push glass out?

 

[PeroK]

If centrifugal force dont exist why glass of water moves outward/toward me when I open fridge door?
Glass of water moves out in relation to door and in relation to earth, so what force push glass out?

Presumably the glass of water is held by the fridge door? What force opens the fridge door?

 

[user079622]

Presumably the glass of water is held by the fridge door? What force opens the fridge door?

yes, glass of water is in door slot, when I open door fast ,glass accelerate toward me...

 

[PeroK]

yes, glass of water is in door slot, when I open door fast ,glass accelerate toward me...

Is that a surprise?

 

[user079622]

Is that a surprise?

If centrifugal force dont exist then yes it is surprise..If centrifugal force exist then it is not surprise.

 

[Lnewqban]

... glass of water moves outward/toward me when I open fridge door?
Glass of water moves out in relation to door and in relation to earth, ...

Does the glass of water slide in direction that is parallel or perpendicular to the door?

 

[PeroK]

If centrifugal force dont exist then yes it is surprise..If centrifugal force exist then it is not surprise.

I fail to see the relevance of centrifugal force. The glass is essentially part of the door and moves with the door. It has no choice, unless it breaks away from the door.

Centrifugal force usually relates to the apparent outward force when an object is not constrained to move in a circle.

What you have in your fridge door is a real outward force on the door handle. I can see no sense in assuming that force does not exist.

 

[user079622]

Does the glass of water slide in direction that is parallel or perpendicular to the door?

View attachment 334805

what force push glass of water out from center of rotation if centrifugal force dont exist?

 

[PeroK]

what force push glass of water out from center of rotation if centrifugal force dont exist?

The contact force with the fridge shelf, which holds the fridge contents in place.

 

[user079622]

Does the glass of water slide in direction that is parallel or perpendicular to the door?

parallel, if you put glass of water at inner part of shelf ,when open door too fast ,glass will slide out

 

[PeroK]

parallel, if you put glass of water at inner part of shelf ,when open door too fast ,glass will slide out

Ah, so you want to know why the glass slides along the shelf?

That's still the contact force(s) between the glass and door. The direction "outwards" changes as you open the door. The initial force is outwards, relative to the half open door. Or, at least, has an outwards component.

That can be tricky to visualise until you look at the forces in polar coordinates.

 

[user079622]

I fail to see the relevance of centrifugal force. The glass is essentially part of the door and moves with the door. It has no choice, unless it breaks away from the door.

glass is sliding along door shelf, from inner part toward outward part if you open door too fast..

The contact force with the fridge shelf, which holds the fridge contents in place.

But why this force has outward direction if we say that centirufgal force dont exist?

 

[kuruman]

Consider a bead with a rod through it (see figure below). When rod rotates as shown, the bead will move away from the axis of rotation.

If you are a non-believer, you will say that the bead will tend to stay in place while the rod rotates through it.
If you are a believer, you will say that opening the door creates a centrifugal force the pushes the bead away from the axis.

The glass on the refrigerator door is a similar case.

Non-believers are often called observers in an inertial frame.
Believers are often called observers in a rotating frame.
Both kinds are good people.

 

[PeroK]

glass is sliding along door shelf, from inner part toward outward part if you open door to fast..

But why this force has outward direction if we say that centirufgal force dont exist?

See my latest post.

 

[user079622]

Ah, so you want to know why the glass slides along the shelf?

That's still the contact force(s) between the glass and door. The direction "outwards" changes as you open the door. The initial force is outwards, relative to the half open door. Or, at least, has an outwards component.

That can be tricky to visualise until you look at the forces in polar coordinates.

Outward force in inertial frame?

View attachment 334806

your glass has zero right movement(left case), that is not correct
right case is correct.

 

[PeroK]

Outward force in inertial frame?

Yes.

 

[user079622]

Yes.

In inertial frame centrifugal force dont exist, so how can we have outward force?

 

[Dale]

If centrifugal force dont exist

The centrifugal force does exist in a rotating reference frame.

In inertial frame centrifugal force dont exist, so how can we have outward force?

It doesn’t accelerate outward in an inertial frame. Why would there need to be an outward force when it doesn’t accelerate outward?

 

[Orodruin]

In inertial frame centrifugal force dont exist, so how can we have outward force?

You don’t. Any motion in the outwards direction is due to tangential acceleration followed by the door rotation resulting in that tangential velocity becoming radial due to the rotation.

 

[user079622]

It doesn’t accelerate outward in an inertial frame. Why would there need to be an outward force when it doesn’t accelerate outward?

Where is accelerate?
You want to say that glass dont have right acceleration as I show in right case?

 

[PeroK]

In inertial frame centrifugal force dont exist, so how can we have outward force?

The initial force is "outward". If we say that the door is initially in the x direction and the initial force is in the y direction. Then that force can generate motion in the y direction. Once the door has rotated to an open position, then the initial y direction is now partly along the length of the door - partly in the "outward" direction.

In other words, the direction you call "outwards" changes with time. And the motion of the glass from the forces on the door becomes "outward" as the direction you call "outward" changes.

See also the example of bead on the rod.

In either case, the "outward" motion is caused by a lack of centripetal force to ensure the object moves in a circle. Friction with the door can provide some centripetal force, but not enough if you open the door quickly.

 

[user079622]

You don’t. Any motion in the outwards direction is due to tangential acceleration followed by the door rotation resulting in that tangential velocity becoming radial due to the rotation.

any graph of this forces ?

 

[russ_watters]

If centrifugal force dont exist...

If you're commenting on it being a "fictitious force", that's not the same as "does not exist".

https://en.m.wikipedia.org/wiki/Fictitious_force

 

[Dale]

Where is accelerate?
You want to say that glass dont have right acceleration as I show in right case?

View attachment 334816

That is correct. In that drawing there is clearly no outward acceleration at any point. Outward acceleration in an inertial frame would mean that the path would be concave outward. But the path is concave inward at each point. Therefore there is clearly no outward acceleration at any point do no outward force is needed at any point.

 

[user079622]

That is correct. In that drawing there is clearly no outward acceleration at any point. Outward acceleration in an inertial frame would mean that the path would be concave outward. But the path is concave inward at each point. Therefore there is clearly no outward acceleration at any point do no outward force is needed at any point.

But what is this outward force in inertial frame that other members talk about?
I dont understand where it comes from..

 

[Dale]

But what is this outward force in inertial frame that other members talk about?

There is no outward force in the inertial frame. If there were then path would be concave outward.

 

[user079622]

There is no outward force in the inertial frame. If there were then path would be concave outward.

"Any motion in the outwards direction is due to tangential acceleration followed by the door rotation resulting in that tangential velocity becoming radial due to the rotation."

Can you explain this ?

 

[Vanadium 50]

OP, I see that you are responding in seconds - sp fast you don't even have time to use capital letters, I suggest that things will go faster for you if you took a few minutes to read and think about the messages people are sending you before responding.

 

[Orodruin]

right case is correct.

No, it isn’t. For several reasons. Nor is the left picture except in very particular cases.

"Any motion in the outwards direction is due to tangential acceleration followed by the door rotation resulting in that tangential velocity becoming radial due to the rotation."

Can you explain this ?

Consider the closed door starting to open. This gives a force perpendicular to the door, which starts putting the glass into motion downwards in your picture. At the insrant of opening downwards will be tangential and not radial. The glass will continue the induced downward motion unless acted upon by another force. However, as the door swings open by even a minor angle. This means that downwards is no longer purely tangential, it has a radial component. Hence, the glass now has a radial component of velocity without ever being acted upon by a radial force.

 

[Lnewqban]

what force push glass of water out from center of rotation if centrifugal force dont exist?

The door would move linearly toward you (assuming the door handle is located by its CG), together with the glass of water, as your force is simultaneously applied on both bodies and directed toward you.

That theoretical linear movement of the door is restricted by the hinges, and it instead rotates about the vertical line of the hinges.

That theoretical linear movement of the glass of water is not restricted by the door's hinges, therefore, it tends to move linearly toward you.
At the same time, the points of the door in non-solid contact with the glass describe a circular motion, which deviates the sliding glass from that theoretical linear movement.

Hence, the glass describes a trajectory that is between circular (hinged door) and linear (non-hinged door).
Please, see animation in post #6 above.