The key to understanding why, for example, a sliding box slows down (seemingly on its own) is to first understand that a net external force acts on the box to make the box slow down. You have probably noticed that a moving object will usually slow down and stop unless some effort is made to keep it moving. (Recall that constant velocity means that the body moves in a straight line and at a constant speed.)Īt first glance, this law may seem to contradict your everyday experience. A body in motion tends to remain in motion at a constant velocity unless acted on by a net external force.A body at rest tends to remain at rest.Newton’s first law of motion states the following: Emphasize the direction of the force of friction.
Ask students to give different examples of systems where multiple forces occur. Ask them where friction may be useful and where it may be undesirable. Ask students to give examples of smooth and rough surfaces. Talk about different pairs of surfaces and how each exhibits different levels of friction.
Discuss examples of Newton’s first law seen in everyday life.