What makes a door move, the torque or the force or both?

[alkaspeltzar]

I have seen an example of a horizontal beam on a hinge. Beam of length L acted on my gravity MG at center, where the hinge supplies 1/4MG force to the door. Therefore there is a net torque, causing angular acceleration and a net force, causing translation about the center of mass.

Maybe it is because many examples focus only on the torque, but i read examples where they only look at the torque on a interior house door say a person pushes it. But if i am not mistaken when we push a door, similar to the beam, there is both torque and force which is why it moves? Without some force, it would have no torque, but it need a force to move its center of mass as well.

Maybe i have taken the force at the hinge point for granted, as with wheels and gears, it is equal to the applied force, so it creates no torque and it can safely be ignored.

Attached is an example of what i reading and applying to the door.

 

[Vanadium 50]

How do have torque without force?

 

[kuruman]

Try to open a door by applying a force at the hinges. If the door opens, it's the force that opens it. If the door doesn't open it's the torque. Why do you think door manufacturers routinely (and cleverly) place door handles where they do?

 

[alkaspeltzar]

Try to open a door by applying a force at the hinges. If the door opens, it's the force that opens it. If the door doesn't open it's the torque. Why do you think door manufacturers routinely (and cleverly) place door handles where they do?

No, i guess my question was i thought the door only had a net torque, no net force. I thought the hinge supplied an equal force to the applied force, so there was only torque doing the work

 

[alkaspeltzar]

How do have torque without force?

Not arguing that. I just thought the door would have no net force. I thought the hinge would supply equal and opposite force, so it was the torque only. But i see that is not true.

There is both a net force and net torque that get it moving correct?

 

[alkaspeltzar]

So from what i understand, since this is not a wheel or gear where the system is balance by an equal force from an axle so the CofMass doesn't move, there has to be BOTH net force and net torque to make a door swing correct?

 

[jbriggs444]

No, i guess my question was i thought the door only had a net torque, no net force. I thought the hinge supplied an equal force to the applied force, so there was only torque doing the work

Possibly you need to consider the distinction between a revolving door and one which is hinged on the side.

Does the center of mass of the revolving door move when the door is used?
Does the center of mass of the hinged door move when the door is used?

Pick an axis about which to compute angular momentum.
Does the angular momentum of the revolving door change when it is used?
Does the angular momentum of the hinged door change when it is used?

 

[alkaspeltzar]

Possibly you need to consider the distinction between a revolving door and one which is hinged on the side.

Does the center of mass of the revolving door move when the door is used? no
Does the center of mass of the hinged door move when the door is used? yes

Pick an axis about which to compute angular momentum.
Does the angular momentum of the revolving door change when it is used? yes
Does the angular momentum of the hinged door change when it is used? yes

So becuase there is both change in angular moment and the c of mass moves in the door hinged(not revolving around a center), there is both net force and torque correct? I feel like i didnt think about it before which is why it is odd. So please let me know

 

[jbriggs444]

So becuase there is both change in angular moment and the c of mass moves in the door hinged(not revolving around a center), there is both net force and torque correct?

Yes. That is the correct conclusion that you should draw.

 

[kuruman]

So from what i understand, since this is not a wheel or gear where the system is balance by an equal force from an axle so the CofMass doesn't move, there has to be BOTH net force and net torque to make a door swing correct?

"Move" is not a good word choice. If the CoM accelerates, there has to be a net force acting on it and you can always pick a point in space about which there will be a net torque.

 

[alkaspeltzar]

"Move" is not a good word choice. If the CoM accelerates, there has to be a net force acting on it and you can always pick a point in space about which there will be a net torque.

Yes, you are right. but i was trying to define the problems such that it was understood there are both types of motion. Rotational about the pivot but still translation thru the center of mass. So in this case, we have a force pushing the door forward but due to the fixed hinge, it is also causing a torque/rotation about the hinge.

 

[alkaspeltzar]

Yes. That is the correct conclusion that you should draw.

why do i feel like in many problems we ignore the hinge force or even the net force on the door, only caring about the torque? Is this because we can figure out how fast the door swings and such without it, so it is not necessary? IF we know the force to the door, we can run the torque calc to determine what torque makes it swing. The fact there is a net force is true, but not helpful in understanding its motion.

Or is it because in many torque examples, say a wrench and a bolt, we only care about the torque on the bolt, and what happens to the wrench is just extra?

 

[jbriggs444]

why do i feel like in many problems we ignore the hinge force or even the net force on the door, only caring about the torque?

If we have a clear and easy way to solve a problem, we can use that approach and ignore all of the other ways, regardless of how clear and easy the alternatives might be.

If we can figure out how fast a door opens using an argument based on torque then we need not bother ourselves by summing linear forces to determine how the center of mass moves. We already know how the center of mass must move.

Lagrangian mechanics is something that I have never fully comprehended, but my understanding is that it provides an entirely different perspective on constraint forces (e.g. hinge forces) in problem solving.

 

[A.T.]

So isn't it both the torque and force applied to the door making it move?

You hand makes the door move. Torque and force are quantities used compute how it will move.

 

[Dale]

why do i feel like in many problems we ignore the hinge force or even the net force on the door, only caring about the torque?

A hinge is a device which is designed to provide any arbitrary force but zero torque (about the hinge). You only care about the torque because the hinge force can be whatever it needs to be to satisfy the motion determined by the torque.

 

[sophiecentaur]

Or is it because in many torque examples, say a wrench and a bolt, we only care about the torque on the bolt, and what happens to the wrench is just extra?

The Force that's applied to a wrench will be in a particular direction and very little actual Torque will be applied by your hand unless the wrench is held like a screwdriver. Also, the effect of this force may just move a light workpiece across the bench without turning it at all. Trying to classify what's "really going on" and give it a name is really a bit futile. Force and Torque are both at work in most situations.

If you are ever lucky enough to have a space flight, observe what happens to free objects when you push and pull them. Failing that, try working with a heavy boat and manoeuvring it against another boat. You soon realize that you 'can' apply torque by gripping a rail with both hands but the result is often surprising.

 

[alkaspeltzar]

The Force that's applied to a wrench will be in a particular direction and very little actual Torque will be applied by your hand unless the wrench is held like a screwdriver.

Not sure what you mean, the bolt would receive a lot of torque from the wrench/hand.

But I do agree, torque and force go hand in hand in most applications

 

[alkaspeltzar]

A hinge is a device which is designed to provide any arbitrary force but zero torque. You only care about the torque because the hinge force can be whatever it needs to be to satisfy the motion determined by the torque.

This makes a lot of sense. The hinge helps provide centripetal force to maintain rotation and create the tangential forces on the center of mass so it moves both rotationally and translationally equally.

Makes a lot of sense then why torque is of the main concern

 

[alkaspeltzar]

A hinge is a device which is designed to provide any arbitrary force but zero torque (about the hinge). You only care about the torque because the hinge force can be whatever it needs to be to satisfy the motion determined by the torque.

Pretty sure someone told me this once. I think this was missing in my understanding. Torque creates the rotation and the hinge/pivot causes the rest to balance.

 

[Nugatory]

Yes, you are right. but i was trying to define the problems such that it was understood there are both types of motion.

Fair enough, but the question you‘re asking is not ‘What makes a door move?” but rather “What makes a door start moving?”. The latter implies changes in momentum and angular momentum, which is what torques and forces do.

 

[archaic]

isn't torque defined in terms of force? as in if there's no force, then there's no torque, right?

 

[snorkack]

isn't torque defined in terms of force? as in if there's no force, then there's no torque, right?

Consider the examples of revolving door then.
If you push one side of revolving door and simultaneously pull the opposite side of revolving door with opposite and equal force then there are gross forces causing torque on the door - but there is no net force on door, and therefore no net force on pivot.
Now suppose you just push the door at one point which is the centre of mass of the door leaf.
There will be no torque if there are no hinge forces - for example the hinges are broken so the push turns out to push the door out of the frame rather than turn the door.
Assume the hinges hold. Then does the door translate?
If you push a leaf of the revolving door, the opposite leaf will be moving in opposite direction, so the door does not translate. It means there must be no net force on the door - the push on one leaf must be balanced by equal and opposite hinge forces if you do not balance it by equal and opposite pull.
If the door, however, has one leaf then the fact that it turns shows that there are hinge forces - but the fact that the door translates shows that the hinge forces must not be equal to the push.

 

[sophiecentaur]

Not sure what you mean,

You can't apply a torque without impressing two forces. You arm can do that (to a small extent) by 'twisting'. Otherwise, you use another (reaction) force and the distance between the forces governs the resulting torque.
By applying a torque, you can ensure that there is no net linear acceleration. You can't ensure that with a simple pull / push on a wrench handle.

 

[sophiecentaur]

if there's no force, then there's no torque, right?

If there's only one force there's no torque.
Why are we chasing our tails without drawing any diagrams?

 

[alkaspeltzar]

Consider the examples of revolving door then.
If you push one side of revolving door and simultaneously pull the opposite side of revolving door with opposite and equal force then there are gross forces causing torque on the door - but there is no net force on door, and therefore no net force on pivot.
Now suppose you just push the door at one point which is the centre of mass of the door leaf.
There will be no torque if there are no hinge forces - for example the hinges are broken so the push turns out to push the door out of the frame rather than turn the door.
Assume the hinges hold. Then does the door translate?
If you push a leaf of the revolving door, the opposite leaf will be moving in opposite direction, so the door does not translate. It means there must be no net force on the door - the push on one leaf must be balanced by equal and opposite hinge forces if you do not balance it by equal and opposite pull.
If the door, however, has one leaf then the fact that it turns shows that there are hinge forces - but the fact that the door translates shows that the hinge forces must not be equal to the push.

So given one leaf door, there are both force and torque on the door, as it turns and translates. Is that correct?

With revolving door, there is only rotation therefore no net force

 

[Nugatory]

isn't torque defined in terms of force? as in if there's no force, then there's no torque, right?

Torque is indeed defined in terms of force, but we can have torque even when there is no net force. An example might be spinning a wheel on a shaft by placing my hands on opposite sides of the wheel and pushing with one hand, pulling with other. The center of mass of the wheel stays put so we know that there's no net force on the wheel, but it starts to spin telling us that there is a net torque on the wheel.

If there's only one force there's no torque.

Even when there is only one force there's still torque (in general - we can always choose a point about which to calculate the torque in such a way that it comes out zero). If there is only one force involved then the net force is necessarily non-zero and the center of mass will accelerate along with any rotation caused by the torque

 

[alkaspeltzar]

Torque is indeed defined in terms of force, but we can have torque even when there is no net force. An example might be spinning a wheel on a shaft by placing my hands on opposite sides of the wheel and pushing with one hand, pulling with other. The center of mass of the wheel stays put so we know that there's no net force on the wheel, but it starts to spin telling us that there is a net torque on the wheel.
Even when there is only one force there's still torque (in general - we can always choose a point about which to calculate the torque in such a way that it comes out zero). If there is only one force involved then the net force is necessarily non-zero and the center of mass will accelerate along with any rotation caused by the torque

Can you confirm my question please. Thanks

 

[Lnewqban]

So given one leaf door, there are both force and torque on the door, as it turns and translates. Is that correct?

With revolving door, there is only rotation therefore no net force

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/mi2.html

Imagine a very wide hinge-less door floating out in space, which you are pushing while being supported by a massive object, like the International Space Station.
The closer to the center of mass of that door that your hand perpendicularly pushes it, the closer to a pure translation movement you induce.
As the pushing area is relocated more and more towards one of the edges of the door, the movement that you induce is more a rotation than a translation.

 

[Nugatory]

Can you confirm my question please. Thanks

You mean the question in the title of the thread?

For an ordinary swinging door, the kind that is hinged at one side: Force causes the center of mass of the door to move, torque causes the door to rotate on its hinges.

For a revolving door: the center of mass doesn’t moveso there is no net force on the door. There is a torque, and that’s what causes it to revolve.

 

[A.T.]

So given one leaf door, there are both force and torque on the door, as it turns and translates. Is that correct?

You need neither force nor torque to turn and translate. You need them only to change the rate of turning and translating.

 

[Nugatory]

You need neither force nor torque to turn and translate. You need them only to change the rate of turning and translating.

This is true, and throughout the entire thread just about everyone (including myself, in the posts just above) has been saying “move” when “start moving” would be more accurate.

There is much to love about the English language, but it is not a precision instrument when used naturally.

 

[alkaspeltzar]

You mean the question in the title of the thread?

For an ordinary swinging door, the kind that is hinged at one side: Force causes the center of mass of the door to move, torque causes the door to rotate on its hinges.

For a revolving door: the center of mass doesn’t moveso there is no net force on the door. There is a torque, and that’s what causes it to revolve.

Yes this question. But you have answered it. I guess for many years i have not thought about the force on the center of mass, just assume it was torque. Learn something every day

So given one leaf door, there are both force and torque on the door, as it turns and translates. Is that correct?

With revolving door, there is only rotation therefore no net force

 

[alkaspeltzar]

This is true, and throughout the entire thread just about everyone (including myself, in the posts just above) has been saying “move” when “start moving” would be more accurate.

There is much to love about the English language, but it is not a precision instrument when used naturally.

i think everyone knew what i meant, this is not english forums LOL

 

[alkaspeltzar]

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/mi2.html

Imagine a very wide hinge-less door floating out in space, which you are pushing while being supported by a massive object, like the International Space Station.
The closer to the center of mass of that door that your hand perpendicularly pushes it, the closer to a pure translation movement you induce.
As the pushing area is relocated more and more towards one of the edges of the door, the movement that you induce is more a rotation than a translation.

In the link, it shows for a point mass the different is null, you can explain the motion either thru torque or thru force, as one induces the other.

Same for the door and your space example. The door would have both rotation and translation, so hence the force moves it forward and causes torque.

 

[Dale]

you can explain the motion either thru torque or thru force, as one induces the other.

While this is always true, sometimes it is more difficult to know one or the other. In the case of a door with a hinge we know that the torque about the hinge provided by the forces at the hinge are zero. We do not know the forces at the hinge. So although the motion can be explained either way (in fact a full description requires both), it is much more simple to analyze the torque. From only the torque we can determine the full motion and thereby determine the unknown hinge force. But since the hinge force is unknown in advance it can only be used in this case as an "after the fact" explanation.

That is the reason why door questions and discussions focus on the torque only. Not because the forces are zero, but because they are unknown and adapt to whatever value is needed to satisfy the motion as determined by the torque.

 

[A.T.]

i think everyone knew what i meant, this is not english forums LOL

You keep asking people to confirm if your formulations are correct. What's the point if you don't want to improve them.

 

[alkaspeltzar]

You keep asking people to confirm if your formulations are correct. What's the point if you don't want to improve them.

Who said I wasn't improving, no need to be rude.

 

[alkaspeltzar]

(in fact a full description requires both), it is much more simple to analyze the torque. From only the torque we can determine the full motion and thereby determine the unknown hinge force. But since the hinge force is unknown in advance it can only be used in this case as an "after the fact" explanation.

Agreed, we need both force and torque for full description and understanding of the motion of the door. But I see now from yourself and others that in rotational motion, it is easier to use/focus on the torque since the forces underway are not clear and/or are changing, at the hinge and such. thks!

 

[sophiecentaur]

Even when there is only one force there's still torque

Yes - I could have put that better. What I meant was that , if you want to apply a certain torque, you can't use just one force - unless you know what it is you're trying to apply the torque to - i.e. where you are pushing relative to a fulcrum or the CM. Hence my point about a screwdriver vs a wrench.

i think everyone knew what i meant, this is not english forums LOL

Sometimes it's necessary to put the Transatlantic members right about the use of some words.

 

[snorkack]

So given one leaf door, there are both force and torque on the door, as it turns and translates. Is that correct?

With revolving door, there is only rotation therefore no net force

Precisely.
Of course, you need gross forces to get a torque, but you can have zero net force if you have a pair of opposite and equal forces.
So in case of revolving door, since there is no translation, you know hinge forces must needs be equal and opposite to the force exerted on door. In case of one leaf door, since there is translation, the hinge forces are different (and smaller).

There are two plausible pivot points to consider. Door hinges, or door centre of mass. If hinges do not hold, and you push the door at exactly the centre of mass, there would be no torque, and the door would be pushed out of doorframe without rotating.

 

[nasu]

Sometimes it's necessary to put the Transatlantic members right about the use of some words.

But the Transatlantic-ness is a frame dependent quality. A simple frame translation and you become the Transatlantic neighbour.

 

[sophiecentaur]

But the Transatlantic-ness is a frame dependent quality. A simple frame translation and you become the Transatlantic neighbour.

It's always 'the other guy' who needs educating.

 

[A.T.]

So given one leaf door, there are both force and torque on the door, as it turns and translates. Is that correct?

The correct statement is:
Given a one leaf door, there are both net force and net torque on the door, as its angular velocity changes and its center of mass accelerates.

With revolving door, there is only rotation therefore no net force

The correct statement is:
With a revolving door, only the angular velocity changes therefore there is no net force

 

[alkaspeltzar]

The correct statement is:
Given a one leaf door, there are both net force and net torque on the door, as its angular velocity changes and its center of mass accelerates. The correct statement is:
With a revolving door, only the angular velocity changes therefore there is no net force

A.T.,

I see the clarification. Yes you are correct. It is always net torque or net force that induces accelerations.

You were right, I was being lazy, it easier to explain 1 on 1 when you can work and draw it out.

Thanks for hearing me out and like other confirming my thoughts.

 

[PeterO]

Don't forget that the door, via the hinges, is connected to the wall, and through those hinges the door can be subject to a reaction force of what ever size and direction is needed for the door to move the way it does (it opens or closes?), when you apply the force you do apply, in the direction you applied it, and at the point(s) of contact you chose - so analysis may be far more difficult than you imagine.
before you touched the door, the hinges were already applying an upward force so it didn't fall to the floor, and the upper hinge was perhaps applying an inward force so the door didn't rotate - and perhaps the lower hinge was applying an outward force to prevent the same. If you removed all but one screw from the top hinge, that last screw might "pull out" showing that just one screw was not capable of providing the required force.

 

[cmb]

Torque is indeed defined in terms of force, but we can have torque even when there is no net force. An example might be spinning a wheel on a shaft by placing my hands on opposite sides of the wheel and pushing with one hand, pulling with other. The center of mass of the wheel stays put so we know that there's no net force on the wheel, but it starts to spin telling us that there is a net torque on the wheel.
Even when there is only one force there's still torque (in general - we can always choose a point about which to calculate the torque in such a way that it comes out zero). If there is only one force involved then the net force is necessarily non-zero and the center of mass will accelerate along with any rotation caused by the torque

Yes, a pure torque resulting in pure angular acceleration with no net translation of the CoM of the body to which the forces are applied is always generated by a balance of (at least two) forces, sum total in anyone axis being zero.

Some very complicated answers here that I think miss the question about the door, because it is not a pure angular acceleration. I think the point you were asking is whether the vector sum of forces from a) the door handle, and b) the hinge, are zero?

Answer; not exactly but close. (answer to you title is 'both')

There are two forces on the door when you push/pull the handle; 1) that force, and 2) the force from the hinge. The magnitude and vector direction of the two forces are not quite precisely equal while the vector direction is not quite opposite.

The reason is that the door not only receives a torque sending it into a spin but also receives a translating acceleration as its CoM begins to accelerate. The force on the handle is always very slightly greater than the force on the hinge if the door has angular acceleration greater than the torque-friction is inducing at the hinge.

Take the case of a slammed door; at the instant the door slams shut, there is a force from the retaining edge of the door which is equal and opposite to the forces from the hinge that result in torque. However, the door also has translational inertia too, perpendicular to the aperture, and there has to be an EXTRA impulse from the door edge and hinge which are both in the same direction to decelerate the translational motion.

This then adds summatively to one force negatively and one force being positively (in some datum direction) giving two forces. When a door slams, the force is greater on the closing edge than the hinge, the delta between the two being the force which decelerates the door motion in the direction perpendicular to the opening.

If you watch a door with a loose hinge being slammed, you may notice the hinge jumps 'outwards' momentarily, this being the reversal of the force vector from one direction when being pushed closed then at the moment of the 'slam' through zero and then the force other direction.

A revolving door is quite different as there is no translational momentum. The forces on the central pivot of a revolving door are always equal and opposite at all times as its CoM never moves, unlike a domestic door.

 

[alkaspeltzar]

Yes, a pure torque resulting in pure angular acceleration with no net translation of the CoM of the body to which the forces are applied is always generated by a balance of (at least two) forces, sum total in anyone axis being zero.

Some very complicated answers here that I think miss the question about the door, because it is not a pure angular acceleration. I think the point you were asking is whether the vector sum of forces from a) the door handle, and b) the hinge, are zero?

Answer; not exactly but close. (answer to you title is 'both')

There are two forces on the door when you push/pull the handle; 1) that force, and 2) the force from the hinge. The magnitude and vector direction of the two forces are not quite precisely equal while the vector direction is not quite opposite.

The reason is that the door not only receives a torque sending it into a spin but also receives a translating acceleration as its CoM begins to accelerate. The force on the handle is always very slightly greater than the force on the hinge if the door has angular acceleration greater than the torque-friction is inducing at the hinge.

Take the case of a slammed door; at the instant the door slams shut, there is a force from the retaining edge of the door which is equal and opposite to the forces from the hinge that result in torque. However, the door also has translational inertia too, perpendicular to the aperture, and there has to be an EXTRA impulse from the door edge and hinge which are both in the same direction to decelerate the translational motion.

This then adds summatively to one force negatively and one force being positively (in some datum direction) giving two forces. When a door slams, the force is greater on the closing edge than the hinge, the delta between the two being the force which decelerates the door motion in the direction perpendicular to the opening.

If you watch a door with a loose hinge being slammed, you may notice the hinge jumps 'outwards' momentarily, this being the reversal of the force vector from one direction when being pushed closed then at the moment of the 'slam' through zero and then the force other direction.

A revolving door is quite different as there is no translational momentum. The forces on the central pivot of a revolving door are always equal and opposite at all times as its CoM never moves, unlike a domestic door.

I found the example I posted in my college physics book. I see what you are saying.

There has to be force and torque on a door which hangs on hinges. Without net force, center of mass wouldn't move. Without torque, it wouldn't rotate but would move forwards.

I kinda think of it like Dale mentioned. The torque created set the door in rotation and the net forces must follow from the difference of applied force minus hinge force so that the c of m moves and satisfies f=ma.

Thanks for the insight!

 

[jbriggs444]

Without torque, it wouldn't rotate but would move forwards.

You need to be a little careful about this. You can have rotation (i.e. rotational acceleration of a body) without any net torque on that body. The catch is that a force whose line of action passes through the selected reference axis has zero torque. There is no rule that says you have to choose the center of mass of the door as your reference axis. You can put your reference axis at the position of the door knob if you like.

If net torque (about a chosen axis) is zero then the rate of change of angular momentum (about that axis) is zero. But angular momentum is not the same thing as rotation.

 

[sophiecentaur]

You need to be a little careful about this.

Agreed but also you need to avoid introducing more and more 'examples' and ways of looking at it. If a force is applied anywhere but through the (fixed) hinge of a door then there is a torque. A torque, only can be applied ` (with a 'reaction wheel', perhaps) and that will have the effect of rotating (accelerating) the door around the hinge. There will be a force through the hinge (centripetal), after acceleration has started.

 

[jbriggs444]

Agreed but also you need to avoid introducing more and more 'examples' and ways of looking at it. If a force is applied anywhere but through the (fixed) hinge of a door then there is a torque. A torque, only can be applied ` (with a 'reaction wheel', perhaps) and that will have the effect of rotating (accelerating) the door around the hinge. There will be a force through the hinge (centripetal), after acceleration has started.

If there is a net force, the torque depends on the choice of reference axis. That choice is free.