![]() ![]() ![]() 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. 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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