- The statement "Elements in the same group on the periodic table of elements have similar properties " is irrelevant

- A rock is a kind of object
- Gravity causes objects that have mass / substances to be pulled down / to fall on a planet
- Gravity is a kind of force
- Hence, gravity is the force that causes rocks to be pulled down on the planet

So, gravity is the force that causes rocks to be pulled down on the planet
Which force causes rocks to roll downhill?
- A rock is a kind of object 
- Elements in the same group on the periodic table of elements have similar properties 
- Gravity causes objects that have mass / substances to be pulled down / to fall on a planet 
- Gravity is a kind of force