When we toss a coin, there are equal chances for both heads and tails to turn up. When we toss a coin, there are equal chances for both heads and tails to turn up, and hence we say that the probability of heads/tails is $\frac{1}{2}$.
But what if we know the speed and angular velocity of the coin.Then can we say that the probability for heads/tails reduces or increases or $\not=\frac{1}{2}$ ?
 A: Yes and that's why we toss a coin without fixing its angular velocity or other parameters of motion, because we want to minimise the predictability of the event.
Now let us take an extreme case - if we toss a coin with angular velocity zero (that is equivalent to simply dropping it ) and head facing up, we can always tell that the probability of getting a head is 1.
A: The idea of a clockwork universe was once very popular among scientists. The idea that every process in the universe is governed by a set of deterministic formulas and we need only to (i) understand the physics perfectly and (ii) measure all of the relevant quantities.
In theory, we should be able to predict the outcome of a coin flip and predict the weather one year from today. In practice however, it is much more useful to model many processes using probability due to chaos theory This states that a minuscule change in initial conditions can lead to a massive change in the outcome.
Basically, if you knew the speed and angular velocity of the coin (as well as a perfect spatial map of your surroundings among other things), there would be no need for probability. Realistically, you will never be able to measure the relevant "inputs" with enough precision for this to be useful.
Side note: Probability is also important in quantum mechanics, but this is besides the point here.
