Basic Statics - A weird outcome. Contradictory?

In summary, the conversation revolves around the concept of static friction and its maximum value in relation to normal force. The participants discuss the validity of using the formula Fs = N x μ to calculate friction force in static objects, and come to the conclusion that it is only applicable in the case of moving surfaces. They also explore the idea that the friction coefficient shown in charts represents the maximum value and that it adjusts itself to keep a body in equilibrium.
  • #1
Femme_physics
Gold Member
2,550
1
This is not a HW question! THIS IS A DISCOVERY that I want to explore. I will explain it as follows:

Suppose I have this exercise, where I know the static coeffecient between the beam and the floor is 0.3, whereas in the wall there is no friction. I want to find out if the beam slides, or stays up. That's easy.

http://img703.imageshack.us/img703/4676/fsmaxtest.jpg

So by doing the calculations, I can see the beam doesn't slide. I solved the problem. Now, let's say I want to experiment further. What happens if I put the result of Fs I got when I didn't calculate Fs_max into the Fs = N x μ equation? I think it's a valid thing to do, since it's in the formula. But what happens then?

I'd get a different result for N. Which doesn't make sense, since N must be 30 [N] to resist the weight of the beam.

The friction coefficient sure can't change, since it is constant!

So, what did I just do? Did I find loop in mechanics theory?
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
Femme_physics said:
What happens if I put the result of Fs I got when I didn't calculate Fs_max into the Fs = N x μ equation? I think it's a valid thing to do, since it's in the formula.

Not valid. If the surfaces were moving, you could use such a formula. In case of static objects, we can only get the maximum friction force.

When we are standing on the floor, there is a normal equal to our weight W . The surface has a μ too. Does it mean we are experiencing a horizontal force of μW on our shoes?!
 
  • #3
When we are standing on the floor, there is a normal equal to our weight W . The surface has a μ too. Does it mean we are experiencing a horizontal force of μW on our shoes?!
Of course not. We don't feel friction without moving in the axis it exists.

Not valid. If the surfaces were moving, you could use such a formula. In case of static objects, we can only get the maximum friction force.

How come?

If you recall

http://img62.imageshack.us/img62/7107/frri.jpg

So clearly either the friction coeffecient or the normal force haven't reached their max value. One of them must change. Since it can't be the normal force, as that would defy mechanics, it must be the coefficient: This is my new assumption. The coefficients we see in charts therefor are maximum values. We reach that maximum value of that coefficient only on the verge of movement.

How about that for theory?
 
Last edited by a moderator:
  • #4
Femme_physics said:
What happens if I put the result of Fs I got when I didn't calculate Fs_max into the Fs = N x μ equation?
You will get the minimum N needed to prevent sliding due to Fs.
Femme_physics said:
I'd get a different result for N.
No, it has nothing to do with the actual N.
Femme_physics said:
The coefficients we see in charts therefor are maximum values.
Yes, the static friction coefficient tells you the maximal horizontal force for a given normal force, for a static case.
 
  • #5
Obviously.In every decent book I read,
they clearly state that Fstatic≤μstaticN
and NOT FstaticstaticN

That is,the friction force adjusts itself to keep the concerned body in equilibrium.And the maximum it can be in order to negate the applied forces is given by μstatic times N.

So the coefficient remains constant,but the maximum force of static friction,ie μstaticN is experienced only on the verge of the movement.
 
  • #6
Got it sorted out. Thanks :)
 

FAQ: Basic Statics - A weird outcome. Contradictory?

1. What is Basic Statics and why is it important?

Basic Statics is a branch of mechanics that studies the behavior of objects under the action of external forces. It is important because it helps us understand and predict how structures and systems will respond to different types of forces, such as gravity, wind, or mechanical loads.

2. What is the "weird outcome" or "contradictory" that is often associated with Basic Statics?

The "weird outcome" or "contradictory" often associated with Basic Statics is the fact that a structure can remain in equilibrium even if its individual components are not in equilibrium. This is known as the principle of virtual work, where the internal forces and moments within a structure cancel each other out.

3. How do we determine if a structure is in equilibrium?

A structure is in equilibrium when the sum of all external forces acting on it is equal to zero and the sum of all external moments is also equal to zero. This means that the structure is not moving or rotating, and all forces are balanced.

4. Are there any real-life applications of Basic Statics?

Yes, there are many real-life applications of Basic Statics, such as designing buildings, bridges, and other structures to withstand different types of forces and loads. It is also used in the design and analysis of machines, vehicles, and other mechanical systems.

5. How can we use Basic Statics to improve our lives?

By understanding the principles of Basic Statics, we can design and build structures and systems that are safe, efficient, and durable. This can help improve our everyday lives by ensuring that buildings and infrastructure can withstand natural disasters, machines and vehicles operate smoothly, and everyday objects are designed to be functional and long-lasting.

Similar threads

Replies
13
Views
2K
Replies
6
Views
817
Replies
1
Views
1K
Replies
7
Views
3K
Replies
7
Views
3K
Back
Top