Balance proves gravity isn't a force

[vin300]

consider a weight balance with two unequal masses on the two arms.It comes to rest after tilting a few degrees. If one of the masses really experienced a greater force, then there is no reason why the balance shouldn't tilt 90 degrees

 

[bapowell]

What kind of scale is this that comes to rest after tilting a few degrees? I'm not a scale expert, but I think the kind of scale you are envisioning has a counter-weight on it, that you adjust to achieve equilibrium.

 

[HallsofIvy]

consider a weight balance with two unequal masses on the two arms.It comes to rest after tilting a few degrees. If one of the masses really experienced a greater force, then there is no reason why the balance shouldn't tilt 90 degrees

Only because there is friction in the pivot. If you have a a balance with no (or very little) friction it will tile 90 degrees.

 

[Ich]

What kind of scale is this that comes to rest after tilting a few degrees?

They place the center pivot a little bit higher than the eyes where you attach the pan. (http://visual.merriam-webster.com/images/science/measuring-devices/measure-weight/beam-balance.jpg" ).
That proves that they know how to build balances, but nothing about gravity.

 

[Acut]

I think other posters have already answered it, but I'm curious:
If gravity isn't a force, what is gravity?

 

[apolski]

I think other posters have already answered it, but I'm curious:
If gravity isn't a force, what is gravity?

Gravity is the effect of curvature of space-time rather than a force.

 

[espen180]

Gravity is the effect of curvature of space-time rather than a force.

To add:
In non-intertial reference frames, gravity can be treated as a resulting psuedo-force, on the same level as the centrifugal and coriolis forces.

 

[ZapperZ]

We shouldn't derail this into something it isn't. The OP clearly isn't aiming along this direction, or at least the "evidence" being used clearly doesn't deal with "gravity not a force and merely a curvature of space-time".

This is an elementary problem based on rather incompletely understanding of a situation. Let's not take it beyond that. There are already many threads dealing with gravity within the GR description. Continue that line of discussion in those appropriate threads.

Zz.

 

[Academic]

Is it the friction of the pivot? If that was the case then the friction would have to be a function of the angle right? Otherwise, could it be that the lever arm length changes? hmm...

 

[Acut]

@apolski and espen180: That question was aimed at the OP.
He evidently has some basic conceptual mistakes - let us not confuse him with general relativity, since it will not solve his problem : the body with the largest mass will feel the largest force (or pseudoforce, or whatever).


@OP: Study torque and angular momentum. Than you will see that the more the balance distances from the horizontal, the least will be the tendency to go up to 90 degrees. If the balance has some friction in it - which is likely to be the case - friction will equilibrate that tendency and the balance will be in equilibrium before the 90 degrees. Sometimes, the most stable position isn't achieved because some constraining (normal) or friction forces are in it. The balance that doesn't tilt completely is analogous to a book that doesn't fall because he is on a table.

Have I convinced you, vin 300? Let me give a perhaps simpler reason. You argued that if both masses had different weights, the rod should tilt to 90 degrees, and since that wasn't the case, both masses should have the same weight. Since you can put any 2 masses in that scale, this would mean that everything weights the same. So, by your reasoning, one apple weights the same as 100 apples - clearly, something is wrong.

 

[russ_watters]

Is it the friction of the pivot? If that was the case then the friction would have to be a function of the angle right? Otherwise, could it be that the lever arm length changes? hmm...

It's not friction, it's the lever arm changing due to the geometry of the scale. Ich had it right.

 

[QEDick1918]

Try that experiment in a vacuum. Good luck with that.

 

[pallidin]

Yep. If the pivot is directly inline with the two different masses, the greater mass will cause a full and complete 90 degree tilt.
The pivot in this case is not directly inline, rather above the two masses; thus becomes a "balance"
As russ said, geometry plays the critical role.

 

[pallidin]

Try that experiment in a vacuum. Good luck with that.

A vacuum would have no substantive effect on outcome.

 

[Jimmy Snyder]

I don't know what kind of scale we are talking about. Imagine a beam that is pinned at its center and freely rotates. Imagine two pans are attached at the extrema of the beam so as to rotate freely like cars on a ferris wheel. Then the beam would go vertical. Indeed it would be very difficult to manufacture such a scale so that it didn't go vertical even with no weights on it.

Although friction might help to retard this effect, I doubt it would help much unless the two weights were very similar. I expect that good balance scales reduce the friction as far as is practical anyway. Instead, I expect that they have some mechanical means of preventing the beam from rotating freely beyond a certain angle, while allowing it to rotate freely when less than that angle. In that case, a difference in weight would cause the scale to rapidly reach that limiting angle. Does someone here have access to a balance scale and can confirm these conjectures?

Here is an example of a balance scale.
http://www.findingking.com/popup.aspx?src=images/Product/large/s-757_freeshipping.jpg"

 

[pallidin]

I don't know what kind of scale we are talking about. Imagine a beam that is pinned at its center and freely rotates. Imagine two pans are attached at the extrema of the beam so as to rotate freely like cars on a ferris wheel. Then the beam would go vertical. Indeed it would be very difficult to manufacture such a scale so that it didn't go vertical even with no weights on it.

Although friction might help to retard this effect, I doubt it would help much unless the two weights were very similar. I expect that good balance scales reduce the friction as far as is practical anyway. Instead, I expect that they have some mechanical means of preventing the beam from rotating freely beyond a certain angle, while allowing it to rotate freely when less than that angle. In that case, a difference in weight would cause the scale to rapidly reach that limiting angle. Does someone here have access to a balance scale and can confirm these conjectures?

Here is an example of a balance scale.
http://www.findingking.com/popup.aspx?src=images/Product/large/s-757_freeshipping.jpg"

Clue: The weights are actually BELOW the pivot center.

 

[QEDick1918]

A vacuum would have no substantive effect on outcome.

Yes. I'm talking about the vacuum of space. Gravity is the only force acting on the objects in space. They are in a free-fall. That was my whole point in stating: "good luck" - as in good luck trying to prove gravity isn't a force.

 

[pallidin]

Yes. I'm talking about the vacuum of space. Gravity is the only force acting on the objects in space. They are in a free-fall. That was my whole point in stating: "good luck" - as in good luck trying to prove gravity isn't a force.

Ah, OK

 

[QEDick1918]

Ah, OK

 

[russ_watters]

Sounds like people are still having trouble visualizing this. See this image: http://www.cdc.gov/diabetes/pubs/images/balance.gif

Picture a slight rotation clockwise. When rotating slightly clockwise, the pan on the right gets closer to the fulcrum horizontally while the pan on the left gets further away, thus eventually finding a torque equilibrium.

Note, this is also why using a flexible pole vastly increases stability for a tightrope walker.

 

[e2m2a]

consider a weight balance with two unequal masses on the two arms.It comes to rest after tilting a few degrees. If one of the masses really experienced a greater force, then there is no reason why the balance shouldn't tilt 90 degrees

Your original title, "balance proves gravity isn't a force" belies some confusion of terms.
Gravity and weight are not the same thing, but are interchanged loosely in discussions about gravity. When a body is in free-fall, it is weightless.
When something prevents it from falling at 9.8 meters/sec sq to the center of the earth, weight manifests in the body, which is a force.

One simple experiment you can do to demonstrate this is take a ball which is attached to a spring. The other end of the spring is attached to the inside of a cup. Hold the cup with one hand and let the ball hang from the spring, until it stops oscillating. At this point, it is in equilibrium. The spring elastic force acting upward cancels out the weight of the ball acting downward.

But now, release the cup. What happens? The weight of the ball "vanishes". The only force now acting on the ball is the spring force. As the ball,spring, and cup accelerate to the earth-- weightless-- under the influence of gravity, the spring will pull the ball back into the cup.

 

[Acut]

But now, release the cup. What happens? The weight of the ball "vanishes".

This is wrong. The perceived weight has vanished, but weight keeps acting.

The only force now acting on the ball is the spring force.

Wrong again! The ball is falling, and it's certainly NOT due to the spring force. Gravity keeps acting upon the ball.

 

[QEDick1918]

Your original title, "balance proves gravity isn't a force" belies some confusion of terms.
Gravity and weight are not the same thing, but are interchanged loosely in discussions about gravity. When a body is in free-fall, it is weightless.
When something prevents it from falling at 9.8 meters/sec sq to the center of the earth, weight manifests in the body, which is a force.

One simple experiment you can do to demonstrate this is take a ball which is attached to a spring. The other end of the spring is attached to the inside of a cup. Hold the cup with one hand and let the ball hang from the spring, until it stops oscillating. At this point, it is in equilibrium. The spring elastic force acting upward cancels out the weight of the ball acting downward.

But now, release the cup. What happens? The weight of the ball "vanishes". The only force now acting on the ball is the spring force. As the ball,spring, and cup accelerate to the earth-- weightless-- under the influence of gravity, the spring will pull the ball back into the cup.

Another way to look at it is, a scale does not actually measure your weight per se. It measures the force of upward gravity that balances the downward force of gravity upon your person (in equilibrium).

 

[Acut]

Another way to look at it is, a scale does not actually measure your weight per se. It measures the force of upward gravity that balances the downward force of gravity upon your person (in equilibrium).

Upward gravity?
Upward?
Do we live in the same planet?

A scale measures the normal force - or the tension in the string/spring that is hanging the object. There's no such a thing as "upward gravity" cancelling "downward gravity".

 

[QEDick1918]

This is wrong. The perceived weight has vanished, but weight keeps acting.
Wrong again! The ball is falling, and it's certainly NOT due to the spring force. Gravity keeps acting upon the ball.

Um, sir... have you ever heard of the spring force constant (Hook's Law) ?

And I could be wrong, but the "vanishes" in quotations may be an indicator of a non-literal interpretation. I think I knew what he meant.

 

[Acut]

Um, sir... have you ever heard of the spring force constant (Hook's Law) ?

Yes. I'm not denying there's a spring force in it.
What I'm telling is that it isn't the only force, as e2m2a claimed.

Juuuuust to be nitpicking: it's Hooke's Law, not Hook's Law.

And I could be wrong, but the "vanishes" in quotations may be an indicator of a non-literal interpretation. I think I knew what he meant.

Bear in mind the OP seems just to be beginning to study physics. I don't think he would perceive the subtlety in e2m2a's post. That's why I wanted to be clear.

 

[QEDick1918]

Upward gravity?
Upward?
Do we live in the same planet?

A scale measures the normal force - or the tension in the string/spring that is hanging the object. There's no such a thing as "upward gravity" cancelling "downward gravity".

Brain fart. I meant upward force - not upward gravitational force. I was obviously not focusing on my writing in that post. I am not quite sure if you gathered my original point sans the error, but what I was talking about in Layman's terms is that gravitational and normal forces do cancel out. I am standing on the ground (at rest), for example. There exerts a normal force (contact force) that opposes gravity. They act in opposite directions. Gravity and normal forces do cancel out in an object at rest (like me standing on a scale) or else I would begin accelerating from where I was standing. That's how I always understood it anyway.

 

[QEDick1918]

Yes. I'm not denying there's a spring force in it.
What I'm telling is that it isn't the only force, as e2m2a claimed.

Juuuuust to be nitpicking: it's Hooke's Law, not Hook's Law.


Bear in mind the OP seems just to be beginning to study physics. I don't think he would perceive the subtlety in e2m2a's post. That's why I wanted to be clear.

I'm probably more of a noob than he is. I've only been on this stuff for about a year.

Hooke, yes.

 

[Acut]

@QEDick1918

I surely got what you meant by "upward gravity". However, it was wrong and should be corrected. I know we sometimes write nonsensical things (yesterday, for instance, I posted about the chemistry of silver in a thread about sodium), but we should try to write as accurate as possible, especially when we know that many beginners are reading what we write.

By the way... is you user name QEDick1918 because of Feynman's year of birth?
Cool.

 

[QEDick1918]

@QEDick1918

I surely got what you meant by "upward gravity". However, it was wrong and should be corrected. I know we sometimes write nonsensical things (yesterday, for instance, I posted about the chemistry of silver in a thread about sodium), but we should try to write as accurate as possible, especially when we know that many beginners are reading what we write.

By the way... is you user name QEDick1918 because of Feynman's year of birth?
Cool.

Indeed it is. He's my favorite physicist (RIP). Yeah, I literally was typing and not thinking. I'm actually glad to be corrected. Helps me learn. Except, I'm not sure how long I'll be here. I already have a warning on me for [unintentional] misinformation.

 

[Acut]

@QEDick1918: Don't worry too much about that. You are just beginning to use Physics Forums. Just be more careful with what you post.

@vin300 (OP): Any doubts left?

 

[QEDick1918]

@QEDick1918: Don't worry too much about that. You are just beginning to use Physics Forums. Just be more careful with what you post.

@vin300 (OP): Any doubts left?

I must just stink at explaining myself.

Anyway, I don't want to get too off topic.

 

[D H]

Your original title, "balance proves gravity isn't a force" belies some confusion of terms.
Gravity and weight are not the same thing, but are interchanged loosely in discussions about gravity. When a body is in free-fall, it is weightless. ...

This post is misleading and is dragging things off-topic.

First things first: This thread is about Newtonian gravity. Per post #8, we need to keep discussions of general relativity out of it.

There are many different definitions of weight, e2m2a. Colloquially, and in some places legally (e.g., the USA), weight is mass.

Let's keep that out of this thread as well. That still leaves multiple definitions of the word "weight" in play. The standard definition of weight, sometimes called actual weight is mass times gravitational acceleration. With this definition a body in free-fall is not weightless.

You are assuming a somewhat non-standard definition of weight, often called apparent weight. This is a measurable quantity, for example by using a spring scale or an accelerometer. Nonetheless, this is not the standard definition of weight.

 

[D H]

A scale measures the normal force - or the tension in the string/spring that is hanging the object.

Correct -- if by "scale" you mean "spring scale". This thread is about balance scales, not spring scales.

 

[D H]

A warning to all users: Please do not talk about general relativity in this thread. Doing so will not help the original poster (along with others) who are confused about why a balance beam does indeed balance as opposed to tipping over.

There are several threads in this forum that talk about how gravitation is a real force in Newtonian mechanics but is a fictitious force in general relativity. If you want to discuss that topic, go to one of those threads.

Before one delves into general relativity it is a very good idea to have a very good understanding of Newtonian mechanics. The explanation for the balance problem is simple and has already been stated multiple times. This is a rotational motion problem, easily resolved in a Newtonian sense. The balance results from the fact that the pivot point is above the two masses.

 

[georger]

Consider every mobile that Calder ever made. They are all at rest and among the various mobiles you will see all angles between 0 and 90 degrees depending on the weights at each end and the lengths of the arms. (The arms are free to move so friction is not involved) Not sure its the same question as the balance but the problem is the same: what's the formula for the angle the arm makes (with the horizontal) as a function of the masses and the lengths of the arms. I tried the sum of the torques = 0 but something else is going on.

 

[russ_watters]

Welcome to PF.

Summing the torques is the proper approach. My guess is you're making the mistake of using the standard equation for torque without any transformations. But the equation requires the applied force to be perpendicular to the lever arm and in this case it is not. So you need to use trigonometry to either transform the force or the lever arm so they are perpendicular to each other. My preference would be to transform the lever arm length by calculating the horizontal component of the distance between the applied force's location and the fulcrum.

 

[georger]

Welcome to PF.

Summing the torques is the proper approach. My guess is you're making the mistake of using the standard equation for torque without any transformations. But the equation requires the applied force to be perpendicular to the lever arm and in this case it is not. So you need to use trigonometry to either transform the force or the lever arm so they are perpendicular to each other. My preference would be to transform the lever arm length by calculating the horizontal component of the distance between the applied force's location and the fulcrum.

Excellent! Thats what I was missing. Let me think about this for a bit. Whats not clear is: what is the vector perpendicular to the arm: Is it the vector whose vertical component is Mg or is it the vector in the corrdinate system of the lever arm rotated from the vertical?? Sorry... I'm not a physics student. Thanks again!

 

[russ_watters]

If you want to use the arm as-is, you need to calculate the component of mg perpendicular to it. Mg is the resultant vector, not a component vector. The two legs of the component vector are drawn parallel to and perpendicular to the arm.

It would certainly help you out if you drew yourself a picture. Bear-in-mind, though, that the arms on these devices are not straight lines and/or don't connect directly to the fulcrum, so you'll also need to draw-in and calculate the length and angle of the "effective" arm from the fulcrum to the weight.

 

[georger]

If you want to use the arm as-is, you need to calculate the component of mg perpendicular to it. Mg is the resultant vector, not a component vector. The two legs of the component vector are drawn parallel to and perpendicular to the arm.

It would certainly help you out if you drew yourself a picture. Bear-in-mind, though, that the arms on these devices are not straight lines and/or don't connect directly to the fulcrum, so you'll also need to draw-in and calculate the length and angle of the "effective" arm from the fulcrum to the weight.

I'll give that approach a try...and indeed I have drawn a picture. By the way its easy to make a simple "mobile" which shows the effect: take a stiff wire (galvanized 16 gage works great) put a loop roughly in the middle (wrap the wire around say a philips screwdriver so its easy to slip off) and attach two peices of cardboard to each end. Hang it buy a thread thru the loop and watch it come to rest at some angle. I just want to solve the case of a straight wire although there is a more complicated case that takes into account the weight of the wire and the shape of the wire. There is also a dynamics problem here : push one end down and watch it rock. Whats the equation of that motion (now ignoring friction)? I guess its just two pendulums stuck together

 

[Dadface]

The balance is designed so that its centre of gravity is vertically below the pivot point when the balance is unloaded.For zero load the weight of the balance itself(beam,scale pans etc) exerts zero moment.If a weight is added this will set up a moment which will cause the beam to turn but the centre of gravity of the balance now shifts horizontally away from the pivot point and sets up a moment in the opposite direction.The additional moment caused by the added weight decreases as the beam turns but the moment caused by the weight of the balance increases.

Whoops originally I wrote vertically above when it should have been vertically below.Now corrected.