The free-body diagram shows all of the forces acting on the system of interest. (b) All of the external forces acting on the system add together to produce a net force, F net. The vector f represents the friction acting on the wagon, and it acts to the left, opposing the motion of the wagon. The weight w of the system and the support of the ground N are also shown for completeness and are assumed to cancel. The system of interest is the wagon and its rider. Arrows representing all external forces are shown. (a) Two children push a wagon with a child in it. Different forces exerted on the same mass produce different accelerations. This concept will be revisited many times on our journey through physics. The concept of a system is fundamental to many areas of physics, as is the correct application of Newton’s laws. Sometimes the system is obvious, whereas other times identifying the boundaries of a system is more subtle. (The internal forces actually cancel, as we shall see in the next section.) You must define the boundaries of the system before you can determine which forces are external. Only external forces affect the motion of a system, according to Newton’s first law. Again looking at Figure 1(a), the force the child in the wagon exerts to hang onto the wagon is an internal force between elements of the system of interest. An internal force acts between elements of the system. The two forces exerted by the other children are external forces.
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For example, in Figure 1(a) the system of interest is the wagon plus the child in it.
What do we mean by an external force? An intuitive notion of external is correct-an external force acts from outside the system of interest. Newton’s first law says that a net external force causes a change in motion thus, we see that a net external force causes acceleration.Īnother question immediately arises. A change in velocity means, by definition, that there is an acceleration. Before we can write down Newton’s second law as a simple equation giving the exact relationship of force, mass, and acceleration, we need to sharpen some ideas that have already been mentioned.įirst, what do we mean by a change in motion? The answer is that a change in motion is equivalent to a change in velocity. Newton’s second law of motion is more quantitative and is used extensively to calculate what happens in situations involving a force. It mathematically states the cause and effect relationship between force and changes in motion. Newton’s second law of motion is closely related to Newton’s first law of motion. Apply Newton’s second law to determine the weight of an object.Understand Newton’s second law of motion.Define net force, external force, and system.
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