Concept of force and newton''s laws, Physics



1.   Ancient view objects tend to stop if they are in motion; force is needed to keep something motion. This was a natural thing to believe in because as we see objects stop moving after some time; frictionless motion is possible to see only in rather special situation such as in vacuum.


2.   Modern view objects tend to remain in their initial state; force is needed to change motion. Resistance to changes in the movement is called inertia. More inertia means it is difficult to make a body accelerate or decelerate.


3.   Newton's First Law states that an object will remain at rest or will move with constant the velocity unless acted upon by a net external force. (A non-accelerating reference frame is called an inertial frame; Newton's First Law deals with the inertial frames.)



4.   More is the force the more is acceleration:

1158_Newtons law.png

5.   The greater the mass of a body, the difficult it is to change its state of motion. Larger the mass means larger is the inertia. In other words, larger mass leads to less acceleration:

2144_Newtons law1.png

By combine both the above observations we conclude that:

2288_Newtons law2.png


6.   Newton's Second Law states that


2118_Newtons law3.png


                (or, we can also, write as F = ma).



7.   F = ma is one relation amongst three independent quantities (m, a, F ). For it to be useful, we must have separate methods by which we can measure mass, acceleration, and force. Acceleration is measured by observing the rate of change of velocity; mass is a measure of the amount of matter in a body (such as two identical cars have twice the mass of the single one). Forces (due to gravity, repulsion of two like charges,a stretched spring, etc).

8.  Force has dimensions of [mass] × [acceleration] = M L T -2 . In the MKS system the unit of force is the Newton. It has the symbol N where:

1 Newton  =  1 kilogram.metre/second2 .


9. Forces can be both internal and external. For instance the mutual attraction of atoms within the block of wood is known as internal forces. Anything pushing the wood is an external force. In the application of  F = ma, remember that F stands for the total external force upon the body.


10. Forces are the vectors, and so they must be added vectorally:


G      G      G      G

F = F1  + F2  + F3  + ⋅⋅⋅⋅ 


This means that the components in the xˆ direction should be added separately, those in the yˆ direction separately, etc.

11. The gravity acts directly on the mass of a body - this is a very essential experimental observation due to Newton and does not follow from F = ma. So the body of mass m1 experiences the force F1 = m1 g while the body of mass m2 experiences a force F2  = m2 g , where g is the acceleration with which anybody (may be big or small) falls under the influence of gravity.


13. Newton's Third Law: for every action there is an equal and opposite reaction. More precisely,

FAB  = −FBA , where FAB  is the force exerted by body B upon A whereas FBA is the force exerted by body A upon B. Ask yourself what would happen if this was not true. In that case, a system of two bodies, even if it is completely isolated from the surroundings, would have a net force acting upon it because the net force acting upon both bodies would be FAB  + FBA  ≠ 0.


 14. If the action and reaction are always equal to each other, then why does a body accelerate or goes in motion at all? Students are often confused by this phenomenon.  The answer: in considering the acceleration of a body you should consider only the (net) force acting upon that particular body. So, for instance, the earth pulls a stone towards it and causes it to accelerate because there is net force acting upon that stone. On the contrary, by the Third Law of Newton, the stone also pulls the earth towards it and this causes the earth to accelerate or moves towards the stone. Though, because the mass of the earth is so large, we are only able to see the acceleration of the stone and not that of the earth.

Posted Date: 7/26/2012 2:06:25 AM | Location : United States

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