Just as friction opposes the motion of solid surfaces that are in contact, a force also opposes motion when a solid object moves through a gas or a liquid. This force, called drag force, occurs when solid objects come into contact with the molecules in the liquid or gas.

Drag that occurs when an object is moving through water is called water resistance. Like friction, water resistance is a force that opposes motion. It causes the object moving in the water to slow down and stop. To reduce the effect of water resistance, people design objects to be streamlined in water. Streamlined means it is shaped in a way to reduce drag. 

Boats are objects designed to be streamlined. The bottom of a boat, called the hull, is usually smooth. It is also pointed at the front which allows water to move past more easily. Being streamlined allows boats to move through water more easily and at a greater speed.