Whether an object will slide or whether an object will rotate?

[Femme_physics]

Let's say I apply a horizontal force on an object sitting on a table. So there's friction. I know how to determine whether it will move or not-- I just see whether the applied force is bigger than the friction force.

F > Fs there's movement
Fs < F there's no movement

That I know.

But what if the object will rotate before it moves? How do I know this?(Oops - sorry, please move that to "classical physics")

 

[Mapes]

Instead of comparing forces, compare moments. Know what I mean?

 

[Femme_physics]

Problem is that the moment limit of a support can be much higher than the applied force...unless the moment limit is a known value (where in friction force is just Normal force divided by static coeffecient) I can't compare...right?

 

[Studiot]

Friction (the sort you are talking about) is independant of contact area.

So imagine a brick sat on a table (It's your turn to draw a diagram).

If you push the brick horizontally along the top with increasing force it will either start to slide or it will start to tip about one corner, where all the friction now acts.

It will only tip if there is enough friction to prevent horizontal movement.

There are four forces acting.

The normal reaction at the corner which is acting as pivot
The force of friction also at this corner
The weight of the brick through the CofG
The horizontal pushing force parallel to the top of the brick


Mapes has suggested taking moments

Where would you take moments about and why?
What would happen as the pushing force was applied at a lower and lower level until it was applied at the botom of the brick?

 

[Femme_physics]

Friction (the sort you are talking about) is independant of contact area.

So imagine a brick sat on a table (It's your turn to draw a diagram).

Np - if it ought to help me then I'll do it when I get off work today. :)

If you push the brick horizontally along the top with increasing force it will either start to slide or it will start to tip about one corner, where all the friction now acts.

Ah... I see. So initially, the normal force is in line with the center of gravity, and after it begins to rotate it becomes the rotatory axis. Fair enough. But before you get there, how do you get to the fact it rotated and didn't slide...I wonder... you said:

It will only tip if there is enough friction to prevent horizontal movement.

How do I know what's "enough friction"?

There are four forces acting.

Agreed.

The normal reaction at the corner which is acting as pivot

Yes, but only when we decide it's going to rotate, the normal reaction "moves" to the corner. I'm thinking before that.

The force of friction also at this corner

Same as above.

The weight of the brick through the CofG
The horizontal pushing force parallel to the top of the brick

Agreed.

Mapes has suggested taking moments

Where would you take moments about and why?

At the point that's rotating-- the furthest point from the "push force" that's touching the ground.

What would happen as the pushing force was applied at a lower and lower level until it was applied at the botom of the brick?

Less chance of rotation as the arm of the push decreases, thereby increasing the chances sliding will be force. When it's applied at the bottom, it will 100% slide rather than rotating if the force is enough to overcome Fs.


Thanks for replying...can't wait to continue this :)

-Jane

 

[Studiot]

At the point of tipping you can imagine an infinitesimal lifting of the corner fartherst from the pivot followed by a gradual lifting of the bottom surface until only the pivot corner is left in contact.

Draw your diagram to represent this situation and apply Mapes' suggestion. You do not need to post the diagram I can imagine it.

 

[Femme_physics]

At the point of tipping you can imagine an infinitesimal lifting of the corner fartherst from the pivot followed by a gradual lifting of the bottom surface until only the pivot corner is left in contact.

Yes, yes I can imagine that.

Draw your diagram to represent this situation and apply Mapes' suggestion. You do not need to post the diagram I can imagine it.

Okay, I've done it. After calculating all moments to that point, I had the urge to calculate all moments to the pushing force. So, I came to the conclusion that I just need to compare all moments to the tipping axis point, and all moments to the push force, to see what's stronger?

 

[Phrak]

Instead of comparing forces, compare moments. Know what I mean?

What's a moment?

 

[Femme_physics]

Torque
i.e. a force that wants to rotate something on an axis

 

[Studiot]

You got it dead right. Isn't it a good feeling to be working most of it out for yourself?
Perhaps your diagram looked something like this?

go well

 

[Femme_physics]

Exactly Studiot, thanks for leading me up to it... brilliance teaching as usual... my diagram was far more messy :P you can see the horror here. Yours was general (all unknowns), and pretty-looking, which is ideal. I tried to add figures to see if it makes sense...

(So much for girly handwriting heh...)

 

[Studiot]

Although you are taking Mechatronics, you are doing the right thing to get a good grasp of basic the underlying physics (mechanics and electronics).
This will stand you in good stead in the future; the mechatronics detail you learn today will be old hat in 5 - 15 years but your basic physics will be there for you to cope with whatever is then new.

This has happens many times in the past with other new and rapidly changing disciplines. Those who learned tubes, transistors, at college found their stuff superceeded by integrated circuits.

go well

 

[Femme_physics]

Thanks Studiot - I agree. The most important thing to me is the basics, which is why I made it a rule to myself recently to make a notebook specifically for proofs, pre-calculus stuff, peripheral physics, and the stuff that will never change due to technology. Great input. :) This thread made it to my facebook *grins*

 

[AlephZero]

All of the above is only half the story, since nobody has mentioned the effect of inertia.

Get a brick-shaped object like a packet of breakfast cereals, and work out how to push it along a table so it falls over backwards (in the opposite direction to the way you are pushing).

That is the same basic situation as a biker pulling a "wheelie" - but if you haven't studied dynamics yet, you will have to wait till then to explain it. (I don't feel inclined to write a textbook on dyanmics 101 to explain it here).

 

[Phrak]

What's a moment?

Torque
i.e. a force that wants to rotate something on an axis

Well, yes, but torque is a few folders up the page in the physics section --though I don't think torque has wants and desires for something to turn.

This thread is posted in the engineering section where the rules change to fit the game. Torque is not equal to moment--from what I gather. And also 'moment' seems to depend upon which continent in which it is invoked--and perhaps what decade. What's a moment in engineering circles?

 

[Femme_physics]

All of the above is only half the story, since nobody has mentioned the effect of inertia.

Get a brick-shaped object like a packet of breakfast cereals, and work out how to push it along a table so it falls over backwards (in the opposite direction to the way you are pushing).

That is the same basic situation as a biker pulling a "wheelie" - but if you haven't studied dynamics yet, you will have to wait till then to explain it. (I don't feel inclined to write a textbook on dyanmics 101 to explain it here).

We only going to start dynamics in the next semester. Thanks for the input though :)

Well, yes, but torque is a few folders up the page in the physics section --though I don't think torque has wants and desires for something to turn.

This thread is posted in the engineering section where the rules change to fit the game. Torque is not equal to moment--from what I gather. And also 'moment' seems to depend upon which continent in which it is invoked--and perhaps what decade. What's a moment in engineering circles?

I've accidentally posted it here instead on classical physics.

 

[Phrak]

We only going to start dynamics in the next semester. Thanks for the input though :)

I've accidentally posted it here instead on classical physics.

OK. Then post a free body diagram, so we can see what you think the forces will look like.

 

[Femme_physics]

My question has already been resolved, Phrak :D And I posted my (messy) diagram after Studiot posted his-- but at that point I had a good clue already of what's going on and how to solve it.

Thanks for chipping in though...see you round the corner :)

 

[Femme_physics]

My question has already been resolved, Phrak :D And I posted my (messy) diagram after Studiot posted his-- but at that point I had a good clue already of what's going on and how to solve it.

Thanks for chipping in though...see you round the corner :)

 

[Phrak]

My question has already been resolved, Phrak :D And I posted my (messy) diagram after Studiot posted his-- but at that point I had a good clue already of what's going on and how to solve it.

Thanks for chipping in though...see you round the corner :)

chipping? It's not your thread, but anther that has set me off. I'm very frustrated at the moment concerning ambiguous definitions. The physics folks do alright. The engineers are ...difficult...with poorly defined terms (so what's new about this? how these folks manage to tie their shoes in the morning is beyond me, though I is one.).

 

[Femme_physics]

Oops -- I meant chiming in :)

But np. Since my question was resolved I don't mind the thread hijacked. It's actually a valid concern, I hope you get an answer.