I know "Geology" is a branch of "Natural sciences" and is for studying natural and physical phenomena related to earth. But "Physical Geography/Natural Geography" seems to study exactly the same thing. What is the main difference between these two fields of study?
Physical Geography is a wide area of study - there is many sub-disciplines in this field, such as Biogeography, Climatology, Hydrology, Pedology, Geomorphology (and others as well). Often the scale interesting for the physical geographer is the recent past and future, calculated in thousand of years before today up to a few million (time scale could vary in the recent past (last few years and up to the glaciations and Pleistocene at large). Further, it is focused on surface processes, and not necessarily on rocks, there is a lot more interest in dirt, soils, looses pebbles, and their dynamics of erosion and deposition, the processes affecting the surface. A physical geographer will enjoy hunting for deposits exposure when in the field, so a wall of ancient dirt stratae.
On the other hand, the Geology field is interested in longer period of time, in rocks, often very deep rocks, the internal geodynamics (mantle, core, magma), will consider the disposition of crystals, petrology, geochemistry of rocks. Otherwise, geologist will be interested in the same topics that physical geographer do, but will use different tools, or consider those topics as a factor among many. Geologist may be mingling with the engineering world better than geographers. They will hunt for outcrops in the field, like intact, in situ rocks that can explain something about the site.
To illustrate this, when I go in the field with geologists, I prefer to bring a shovel, they bring a hammer (I bring one too but will choose the shovel over!). We will enjoy the accessibility to a drill to make boreholes, but will be happy with the first 5 meters depth where they would be interested more in deeper drilling. Further, I will be glad to drill in soils and dirt, where they would definitely prefer in rocks.
There is overlap of course, but >90% of most geology courses deal with topics which don't come into physical geography, such as geochemistry, geophysics, petrology and mineralogy, hydrogeology, economic geology,and palaeontology. Some 'Earth Sciences' courses are wider in scope, encompassing meteorology, oceanography and climatology - all of which have differing degrees of overlap with physical geography.
Sedimentology and plate tectonics are generally thought of as geological topics. I wouldn't include these as 'physical geography', but others might.
The topic of physical geography has also changed in scope over the years, now being heavily linked to the nature and practice of 'geographic information systems', and 'remote sensing'. Again there is overlap, but geology is still a distinct range of topics which is separate, complementary to, and much greater than physical geography in scope.
Geology deals with the solid Earth below our feet: composition of rocks, dating of rocks, plate tectonics, identifying natural resources below the ground. It might also deal with the composition of the solid part of other planets - but I am not sure about that.
Phyical geography deals with the planet as a whole, including atmosphere, oceans, ground water, and partly 'what is living on the planet'. Climate modeling can also be considered as a part of physical geography.
Thus, phyiscal geography is broader than geology when it comes to the Earth surface and consideres also the compartments air and water. In contrast, geology goes deeper into the Earth.
From my feeling, geology is more focussed on the past/history of the Earth, whereas physical geography is more about understanding what is happing now and what will happen in future.