![]() ![]() When a body is thrown as a projectile the action of gravity does no work on the horizontal component of the throw, because it is acting at right angles. When θ= 90, cos θ= 0 and F has no component in the direction of motion and so no work is done. When θ= 0, cos θ= 1 and so W= Fs, is at its maximum. See animation above: (when it is on the work part!) If the force does not act in the direction in which the motion occurs but at angle θ to it then the work done is defined as the product of the component of the force in the direction of motion and the displacement in that direction. In the simple case, when a constant force F and the displacement s are in the same direction and we define the work W done by the force on the body by W= Fsįor the above example of holding the weight above the ground, even if it may feel as if a lot of work is being done, because no force is acting on the weight and it isn't being moved in scientific terms no work whatsover is being done. Work is done when a force moves its point of application along the direction of its line of action. However the scientific definiton for work is much more specific and is that work is only done when a force is applied to an object over a specific distance, i.e. ![]() We generally think of something that is hard to do, e.g holding a heavy weight above the ground. A newton-meter is given the special name joule(J), and One joule is not a large amount of energy it would lift a small 100-gram apple a distance of about 1 meter.When we think of work, we think of jobs, chores, toils. Thus, in SI units, work and energy are measured in newton-meters. From the definition of work, we see that those units are force times distance. The drawing shows the latter, with the force from the generator upward on the briefcase, and the displacement downward. The other interpretation is that the generator does negative work on the briefcase, thus removing energy from it. One interpretation is that the briefcase’s weight does work on the generator, giving it energy. There are two good ways to interpret this energy transfer. Finally, in Figure 1(e), energy is transferred from the briefcase to a generator. In contrast, when a force exerted on the system has a component in the direction of motion, such as in Figure 1(d), work is done-energy is transferred to the briefcase. For example, the person carrying the briefcase on level ground in Figure 1(c) does no work on it, because the force is perpendicular to the motion. There must be displacement for work to be done, and there must be a component of the force in the direction of the motion. Here so Why is it you get tired just holding a load? The answer is that your muscles are doing work against one another, but they are doing no work on the system of interest (the “briefcase-Earth system”-see Chapter 7.3 Gravitational Potential Energy for more details). The person holding the briefcase in Figure 1(b) does no work, for example. To examine what the definition of work means, let us consider the other situations shown in Figure 1. Here the work done on the briefcase by the generator is negative, removing energy from the briefcase, because F and d are in opposite directions. (e) When the briefcase is lowered, energy is transferred out of the briefcase and into an electric generator. ![]() Energy is transferred to the briefcase and could in turn be used to do work. (d) Work is done on the briefcase by carrying it up stairs at constant speed, because there is necessarily a component of force F in the direction of the motion. (c) The person moving the briefcase horizontally at a constant speed does no work on it, and transfers no energy to it. No energy is transferred to or from the briefcase. (b) A person holding a briefcase does no work on it, because there is no displacement. Note that F cosθ is the component of the force in the direction of motion. (a) The work done by the force F on this lawn mower is Fd cosθ. Where is work, is the magnitude of the force on the system, is the magnitude of the displacement of the system, and is the angle between the force vector and the displacement vectorįigure 1. ![]()
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