Register to reply 
Help Me Understand Mechanical Advantage Please 
Share this thread: 
#19
Feb1012, 07:20 AM

Sci Advisor
PF Gold
P: 1,776

It is the same force that keeps us from falling to the center of the Earth though gravity is trying to make us do just that. It is also an important consideration when setting up a mechanical advantage system. You can have a strong lever and good advantage but if you use a weak fulcrum it will break before you lift your load. Ultimately to get a full understanding I think one should study vectors and understand how the various directed forces add up and cancel. 


#20
Feb1012, 11:34 AM

Sci Advisor
Thanks
PF Gold
P: 12,171

'Looking back' one can often see a link between elementary and advanced stuff but only when 'you know the answer anyway'. Also, I am not at all happy about the term "Kinetic energy of the Load arm" in the first post. Moments have nothing at all to do with MASS or Kinetic energy. They relate FORCES and DISTANCES. If you consider Power, then you should be discussing Force times Speed  which is measured in Watts, in the end. 


#21
Feb1012, 08:20 PM

P: 389




#22
Feb1012, 10:33 PM

P: 5,630

I think anyone who has ever jacked up a car ought to be able to form the intuitive understanding you ask about. You are doing all this winding of the crank (with a screw jack) covering much distance with your hand, while the car, conversely, moves upward very, very slowly by comparison. The jack is "collecting" all the force you apply and slowly concentrating it to raise the car. With each turn of the crank you apply a certain force over, say, 3.1416 feet (if the crank circle radius is 6 inches, say) and the jack concentrates all that force that was spread out over all that distance to something compact that actually raises the car about 1/8 of an inch. 


#23
Feb1112, 02:31 AM

P: 758

Why do you think that moments has nothing to do with kinetic energy ? Kinetic energy is the energy possessed by a body or an object by virtue of it being in motion. When we apply force on effort arm on one side , then we are applying muscular energy to it. Aren't we ? Energy is conserved and so that energy sets the effort arm in downward motion , say. Being in motion it has to possess kinetic energy. This in turn sets load arm in upward motion , and so it also must possess kinetic energy equal to effort arm due to law of conservation of energy which is a universal law. Moreover , whenever we do work , we say or we mean in Newtonian physics , that if an only if that object covers some displacement. Hence there is momentum in that object and being in motion it must have kinetic energy. Moment of an arm is the turning effect of that arm also known as torque. It is force times perpendicular distance of line of action of force from point of rotation. So its different from the force. Nonetheless , if there is moment (clockwise or anticlockwise) , there is kinetic energy. Right ? Please correct if I am wrong.. __________________________________________________________________ So OP is asking about interaction of molecules ? Then its quite obvious that total weight downward equals sum total of weight of all the atoms downward. So if one side arm is greater than other , then at that side already there are more atoms and hence greater downward weight. So we apply less additional weight for the both side to be in equilibrium. Also centre of gravity will be at midpoint of lever and hence will dominate at side which has greater lever arm. But we assume the lever weight massless in practical applications and in theories of textbooks and in studying laws like law of lever by Archimedes. Do we not ? 


#24
Feb1112, 04:38 AM

Sci Advisor
Thanks
PF Gold
P: 12,171

In my opinion, if you are trying to help someone to understand something about Physics then we are duty bound to use the correct and well defined terms. If we don't then we can confuse them and leave them worse off than they were in the first place.
You want to talk about "kinetic energy of a force"? Well, perhaps you could quote a reputable source in which the term is used. That would be the acid test. I certainly never came across such confusion in a textbook. A lever doesn't have to be moving for the force to be 'multiplied'. No Mass is mentioned in the Principle of Moments. I know there is a method for working out forces in structures that is referred to as 'virtual work', in which you allow the structure to distort by an infinitessimal amount and then discuss the 'force times distance' involved. That does NOT involve the movement of masses (KE). You may have a 'feeling' about this and this may be your personal way to get closer to the subject but please don't assume to be enough of an authority about it to instruct others in this approach. It is basically flawed because it mixes dimensions in a very dodgy way. I will withdraw my objection if you can give me a credible reference, though. 


#25
Feb1212, 11:42 PM

P: 758

Talking about moments or torque it is force times perpendicular distance of line of action of force from point of axis. Of course it has same unit as that of work which is Newton metres in SI system. We also say that 1 Newton metre = 1 Joule But we cannot say that work and moments are same. Thay are entirely different. Yet when lever arm moves downward , say and other arm moves upward , then lever as whole suffers clockwise couple. Yet there is motion in it and so kinetic energy. Is there not ? Sir , please tell me what's wrong in my previous post , post 23 and tell me where that analogy fails. Thanks... 


#26
Feb1312, 03:27 AM

Sci Advisor
Thanks
PF Gold
P: 12,171

Of course there is often an 'association' between Force and Kinetic Energy in machines of all sorts. However, there is more to Physics than mere associations. We try to do better than that. Firstly, I notice that you haven't quoted a reputable reference for the expression "Kinetic energy of a Force" and I am not surprised. The text book you quote from states the 'real' relationships and does not even imply that 'your' phrase is valid. You have constructed your own argument by extending too far from what you have read in the book. One simple way to demonstrate your inappropriate use of KE in the context of levers is to consider a simple 2:1 length lever with balanced masses (Ratio 1:2). If you allow some movement (say the large mass moves downwards) and do some simple calculation for the KE of each mass, you find that the smaller mass will have TWICE the KE of the larger mass. The velocity will be twice and the mass is half, so the mv^{2}/2 is NOT the same in each case. So, although we have done a calculation involving this lever, it really doesn't lead to any useful conclusion and definitely not an equation that is of any use for solving a 'balance' problem. There is some other form of Energy which would need to be considered as well. Another nail in the coffin is to consider Friction. When you turn a wrench against a very sticky thread, your hand / body may have some kinetic energy but what is moving on the other implied end of the lever? A nut, with a mass of just a few grams, rotating very slowly. Any energy that you may be putting into the system is not turning up as identifiable Kinetic Energy (there will be some KE in the form of internal energy  heat  but this is outside of your analysis and doesn't count). Work has been done but KE is not relevant  or only a tiny part of the situation. 


#27
Feb1312, 04:50 AM

P: 5,630




#28
Feb1312, 05:00 AM

Sci Advisor
Thanks
PF Gold
P: 12,171

This is a great example in which someone's own visualisation of a situation, which may work fairly well for them, personally, in many circs should not be used as a tool for 'explaining things' unless it is 'rock solid'. If anyone takes the huge step of 'explaining' something on a forum like this, they are taking on the mantle of TEACHER. God knows, there are enough professional teachers who actually struggle with many of the basics that they are expected to teach and what they have told students can be to blame for a lifetime of subsequent conceptual problems. 


#29
Feb1312, 07:14 AM

Sci Advisor
PF Gold
P: 1,776

If one wants to get super technical, mathematically a torque (or other gen. force) differs from work in that it is a linear operator mapping differentials in one coordinate to differentials in another not a stand alone quantity. You see that in the way force is measured, you have to do a little bit of work over a little bit of displacement to see the ratio. Look explicitly at how a scale or a torque meter works. (or a volt meter, or a pressure meter...) 


#30
Feb1312, 07:52 AM

Sci Advisor
Thanks
PF Gold
P: 12,171




#31
Feb1312, 10:05 AM

P: 758

I found that kinetic energy of load arm was not equal to kinetic energy of effort arm. *Sigh* , this proves that I applied wrong concepts ! Thanks ! Yet I cannot find answer to OP's question. He wants to analyze molecular interactions ? In other words , I think he is looking for theoretical reasoning rather than mathematical deductions. I guess I have the answer but this involves considering of mass of lever. I searched , googled etc. but cannot find answer to his question. Here is my answer ( please correct if wrong) : We know that centre of gravity is the point where total weight of body is supposed to act. Let one arm of lever be longer and other be shorter. Then its quite obvious that total weight downward equals sum total of weight of all the atoms downward. So if one side arm is greater than other , then at that side already there are more atoms and hence greater downward weight. So we apply less additional weight for the both side to be in equilibrium. Also centre of gravity will be at midpoint of lever and hence will dominate at side which has greater lever arm. But we assume mass of lever to be massless , do we not ? Am I correct ? 


#32
Feb1312, 11:24 AM

P: 5,630

He understands that it does make a difference, and he knows the formula, he doesn't understand why it makes a difference. Since he knows the formula, restatements of the formula are uninformative to him. He wants a simply stated explanation of the reason. 


#33
Feb1312, 11:48 AM

Sci Advisor
PF Gold
P: 3,668

Have your little brother stand on a 2X4 and lift him at various distances from his feet.. 


#34
Feb1312, 12:30 PM

P: 5,630




#35
Feb1312, 02:08 PM

P: 4

thanks. I'll try to understand this. i tried crowbarring some nails before starting thread..I won't give up now knowing that it IS understandable that the greater distance but less force on my end of the lever will move an object with all the concentrated force but for a shorter distance.



Register to reply 
Related Discussions  
Mechanical Advantage of a Lever  Introductory Physics Homework  4  
Mechanical Advantage  Mechanical Engineering  1  
Mechanical Advantage :  Introductory Physics Homework  1  
Mechanical Advantage  Introductory Physics Homework  2  
Mechanical advantage  Introductory Physics Homework  10 