If classical mechanics is flawed,why do we still use it?

Answer 1

Because it still works for any phenomena you can observe with the naked eye. You can still use it to build bridges, design automotive suspensions, construct skyscrapers, and even do a reasonably good job of calculating trajectories for spacecraft (as long as you include gravitational attraction in your definition of classical mechanics.)

Where classical mechanics fall down is in the super-small -- starting with the whole "is light a particle or a wave?" question from about a hundred years ago. (Look up the Michelson-Morley experiment to see one of the outcomes of that question.) And at the subatomic level, there are phenomena that classical mechanics can't explain -- how particles separated from one another seem to "know" what happens to each other, implying some kind of "action at a distance," the repudiation of which led to Newton's original work on classical mechanics, including the three laws of motion and the theory of gravitation. Of course, we know that "action at a distance" can be explained by one of several forces which are invisible but whose effects are plain: gravitation, magnetism, and the strong and weak atomic forces.

But basically, if you can see it without a microscope, classical mechanics does an adequate job of describing -- and more important, predicting -- what you're looking at.

Answer 2

Classical mechanics is not 'flawed' as such. It is a special case, an approximation of the much more general relativistic mechanics.

Let me give you an example-
We know that the mass of a body does not change when it moves (according to classical mechanics) and indeed at our every day 'extreme' speeds it doesn't.

but the actual expression for the mass of a body is:

m = m0 / ( 1 - (v/c)^2 )^0.5
where m0 is the rest mass
c is the speed of light
v is the body's velocity

You see! The change in mass is apparent only when we approach sufficiently high fractional values of c.
i.e v > c/100

and as c= 3 X 10^8 m/s
I don't think its possible in the immediate future!!!!

Answer 3

An excavator is more productive in digging than a hand spade, it’s true! but I would prefer my spade in a small kitchen garden instead of excavator. So classic physics is for our daily use, and Einstein for particles in the labs.

Answer 4

It is only flawed at extreme boundary conditions, such as near the speed of light, and at very, VERY small scales.

For everything in between, it is an excellent model for what happens in the real world. "Excellent" as in: the error is so small that it can be completely ignored.