Think of friction as a resistance to motion that slows things down.Ĭonsider an air hockey table. Drag opposes and slows the motion of an object through a fluid (see Figure 4.3). Rolling resistance impedes the rolling of a wheel. When the friction prevents the object from sliding, it is called static friction. Kinetic friction is a force by a surface parallel to the surface that opposes the motion of a sliding object and causes it to slow down. We further identify different types of friction. Generally, friction is an external force that acts opposite to the direction of relative motion or to prevent slipping. What force acts on the box to slow it down? This force is called friction. Without this net external force, the box would continue to slide at a constant velocity (as stated in Newton’s first law of motion). 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.

Moment of inertia equation for objects