The tidal force or tide generating force is a secondary effect of the force of gravity and is responsible for the tides. In other words, the forces that cause the tides are called the tide-generating forces. It arises because the gravitational acceleration experienced by a large body is not constant across its diameter. One side of the body has greater acceleration than its center of mass, and the other side of the body has lesser acceleration. The Moon's (or Sun's) gravity differential field at the surface of the Earth is known as the tide generating force. When a body (body 1) is acted on by the gravity of another body (body 2), the field can vary significantly on body 1 between the side of the body facing body 2 and the side facing away from body 2. This causes strains on both bodies and may distort them or even, in extreme cases, break one or the other apart. These strains would not occur if the gravitational field is uniform, since a uniform field only causes the entire body to accelerate together in the same direction and at the same rate. The Moon's (or Sun's) force of gravity caused by an object gets weaker as one move farther away from that object. The Earth is pulling the Moon, and the Moon is pulling the Earth. The Moon pulls more strongly on the side of the Earth facing the Moon than on the side facing away from the Moon. Because the gravitational force on one side of the planet is different from that on the other side, it is called a tidal force. Because planets are not perfectly rigid, they deform when subjected to such tidal forces. They deform as if they are being pushed from the top and bottom, and a bulge forms on either side of the planet. These two bulges are called tides. On Earth, near the ocean, these tides can actually be seen. The ocean water rises high along the beach, twice each day. If a body is very rigid or is not held together well, instead of getting pushed and pulled out of shape, the tidal forces can actually tear the body in half. This is the primary mechanism that drives tidal action and explains two tidal equipotential bulges, accounting for two high tides per day.
