Physics is the field of science related to motion, gravity, speed, and much more. Position is used in physics to determine the motion of an object. Velocity includes how fast an object is moving, or its speed, and its direction of motion. Speed can be determined using different measurements over a period of time. The change in velocity is called acceleration. Gravity and friction are examples of forces which affect the speed or velocity of an object’s motion. Gravity causes objects to be attracted to other objects, for example to the earth. The objects are “pulled” to the Earth’s surface by the force of its gravity. Friction is a force that opposes motion. It acts between two objects that are in contact.