A force is needed to change the motion of an object. Stopping a moving object, starting to move an object that is at rest, speeding up or slowing down an object all require unbalanced forces. Students should be aware that the more massive an object is the more force is needed to change its motion. In the absence of any unbalanced forces, once an object is in motion, it will continue in motion. Friction is a force that opposes the motion of an object or the attempt to move an object.
Friction is the force that causes objects to stop moving once they are in motion.
Friction can also produce motion, as in the case of walking. Usually heat is produced as a result of friction. Some surfaces are more difficult to move objects over. Examples include dragging a box across a carpet Vs a smooth floor. Gravity is a force of attraction that pulls objects toward the earth.