Suppose you are hitting a ball with a bat and then using Newtonian or classical mechanics you can tell how long the ball will stay on air, how far it will go and trajectory of the ball. Even you can calculate at which speed the ball will hit the ground. To answer all the questions you will need to do some math. You will also need some data: mass of the ball, acceleration due to gravity, at which momentum & angle the ball was hit. And to be more accurate you will also need to know the air speed and air resistance.
What about molecules, atoms and subatomic particles like electrons, protons and neutrons?
You can imagine hitting an electron with a very tiny bat and then you are trying to calculate how far the electron will go and trajectory of the electron. Unfortunately it’s not possible using formulas derived in Newtonian or classical mechanics. There are some problems: in case of a ball you can precisely tell the initial position, speed and momentum of the ball. But according to Uncertainty principle it’s impossible to measure particle’s position and momentum both accurately. You can say probability of finding the electron in that place is high. So when you don’t know the initial position and momentum of the electron accurately, it’s impossible to say how far an electron will go after hitting with an imaginary tiny bat.
Bohr Theory of atom
We all know that at one stage it was a successful theory but it could only explain single electron hydrogen atom and single electron ions like He+ and Li2+. Bohr model will tell you that, at ground state radius of the electron orbit is precisely 5.3×10-11 m. But quantum theory which led to quantum mechanics will say different experiments will give different results but most probable value is 5.3×10-11 m.
So what do you think? Quantum mechanics is not an accurate mechanics?
Actually reverse is true, classical mechanics is an approximate version of quantum mechanics. Classical mechanics can’t explain interaction between subatomic particles but quantum mechanics can. Let’s go back to the ball again. It consists of so many atoms and deviation from its average value is very little to notice. This is why Newtonian or classical mechanics (which is an approximate version of quantum mechanics) works very well. In fact in this case using quantum mechanics will make it very complex.
We can easily guess why classical mechanics is called so, but what about quantum mechanics?
In 1900 when classical physics was failing to explain blackbody radiation German physicist Max Planck said radiation isn’t emitted continuously. Radiation is emitted in discrete packets called quanta. Quantum is singular form of quanta. Actually this is considered the beginning of quantum theory.
Quantum mechanics helps scientists to understand how particles make atoms. Without quantum mechanics nuclear energy or semiconductor industry couldn’t be developed.
