The system of interest is the wagon plus the small child, as shown in part (b) of the figure. It is important to define the system at the beginning of a problem to figure out which forces are external and need to be considered, and which are internal and can be ignored.įor example, in Figure 4.4 (a), two children push a third child in a wagon at a constant velocity. A system is one or more objects that you choose to study. A more precise definition is that it acts on the system of interest. Recall from the section on Force that a net external force acts from outside on the object of interest. (The coefficient of friction is discussed in more detail in another chapter, and the normal force is discussed in more detail in the section Newton's Third Law of Motion.) Where μ is the coefficient of friction and N is the normal force. Whenever you encounter the phrase at constant velocity, Newton’s first law tells you that the net external force is zero. So if the man applies +50 N of force, then the force of friction must be −50 N for the two forces to add up to zero (that is, for the two forces to cancel each other). The mathematical way to say that no net external force acts on an object is F net = 0 F net = 0 or Σ F = 0. Why? According to Newton’s first law of motion, any object moving at constant velocity has no net external force acting upon it, which means that the sum of the forces acting on the object must be zero. (The positive sign indicates that, by convention, the direction of motion is to the right.) What is the force of friction that opposes the motion? The force of friction must be −50 N. A man pushes a box across a floor at constant velocity by applying a force of Additionally, if we know enough about friction, we can accurately predict how quickly objects will slow down. On a frictionless surface, the puck would experience no net external force (ignoring air resistance, which is also a form of friction). With friction almost eliminated, the puck glides along with very little change in speed. However, when the air is turned on, it lifts the puck slightly, so the puck experiences very little friction as it moves over the surface. When the air is turned off, the puck slides only a short distance before friction slows it to a stop. 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.
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