Gravity is the force that gives weight to objects and causes them to fall to the ground when dropped. It also keeps our feet on the ground. You can most accurately calculate the amount of gravity on an object using general relativity, which was developed by Albert Einstein. However, there is a simpler law discovered by Isaac Newton that works as well as general relativity in most situations.
Newton's Law of Gravitation
Newton's law of universal gravitation states that the gravitational force between two objects is proportional to the mass of both objects, divided by the square of the distance between the objects. Or more simply: Gravitational force=(G_mass1_mass2)/(square of distance), where G is Newton's gravitational constant. You can use this law to work out how much gravity is on an object.
Mass of Objects
The mass of the two objects, denoted in the above equation as mass1 and mass2, is the first factor that affects the amount of gravity that acts on each of the objects. The larger the masses, the more gravitational force each object exerts on the other. Put simply: the more mass an object has, the more gravity acts on that object.
Distance Between Objects
The second factor that affects the amount of gravity on each object is the distance between the two objects. The larger the distance, the less gravitational force each object exerts on the other. This means that the closer one object is to another, the more gravity acts on that object.
Due to the large size and mass of Earth, a simplified version of Newton's law can be used to determine the amount of gravity on an object. In this case, the gravitational force is equal to the mass of the object multiplied by the gravitational acceleration caused by Earth: Gravitational force = mass*g, where g is the gravitational acceleration and is equal to 9.81 meters per second squared. On Earth, mass is the only factor that affects how much gravity is on an object. Gravity affects an object with a large mass more than it affects an object with a small mass.
The same formula can be used for finding the amount of gravity acting on objects on other planets and moons, but the gravitational acceleration will be different for each planet or moon.