Talk:Physics Study Guide/Force
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Observations below are absolutely correct and need to be adopted.
Sure kr06 09:53, 7 January 2006 (UTC)sunil
I have already edited this page (a few minutes ago) in order to clarify two equations given on this page. The first one concerns the F = ma form of Newton's Second Law. This is a standard form which appears in most basic textbooks, but it is not the general mathematical form of the Law, because it assumes that the mass is constant. Consequently, I have added some comments to clarify this.
The second clarification concerns the form of the Second Law in terms of change of momentum. In the equation as originally given, the delta p was divided by time; this should actually be "change in time" or "elapsed time". Consequently, I modified the equation to reflect this. I also clarified the comment about the "equality" of these two forms of Newton's Second Law and the statement concerning acceleration being the rate of change of velocity.
The basic reasons for these edits is that the originals were fundamentally in error, and needed to be corrected. I hope the changes make the subject clearer for the reader, as well as being more correct.
- Would it be helpful to give some examples of forces ? Maybe even define the term "force" ? In particular, I want to avoid giving the false impression that the definition of force is "mass times acceleration". For example, I can push on the bumper of my car with quite a few Newtons of force. But if the parking brake on my car is set, it's not going to move ( a = 0 m/s^2 ). If I just blindly plug numbers into the f=ma equation, I get roughly (20 Newtons) = (1 000 kg)*( 0 m/s^2), which is nonsense. (perhaps we should emphasize that the f=ma formula is the definition of *mass*).
It's true that 
is really the exact formula. This is approximately equal to F=ma only at speads much less than the speed of light.
Some examples would really clarify the formula f = ma!
The statement about Einstein should be removed. He can hardly have had anything bad to say about the third law, which just expresses that momentum is conserved. As for the first and second, they're still fine of you adapt the definition of momentum to p = (gamma) m v.
