Consider a force applied parallel to the surface that moves an object with constant acceleration.įrom the definition of work, from Newton's second law of motion, and from kinematics, W = Fx = max and v f 2 = v o 2 + 2 ax, or a = ( v f 2 − v o 2)/2 x. The expression for kinetic energy can be derived from the definition for work and from kinematic relationships. Kinetic energy is the energy of an object in motion. For a gradually changing force, the work is expressed in integral form, W = ∫ F For example, in this case, the work done by the three successive forces is shown in Figure 1. The total work done is the total area between the curve and the x axis. The work performed on the object by each force is the area between the curve and the x axis. The force is increasing in segment I, is constant in segment II, and is decreasing in segment III. Figure shows the force‐versus‐displacement graph for an object that has three different successive forces acting on it. If work is done by a varying force, the above equation cannot be used. The SI unit for work is the joule (J), which is newton‐meter or kg m/s 2. x, where F is the applied force and x is the distance moved, that is, displacement.(However, work was done to set the mass in motion.) Mathematically, work is W = F
The centripetal force is directed toward the center of the circle and, therefore, is not moving the object through a distance that is, the force is not in the direction of motion of the object. Another example of the absence of work is a mass on the end of a string rotating in a horizontal circle on a frictionless surface. Work, according to the physics definition, is being accomplished while the heavy object is being lifted but not while the object is stationary. A person that holds a heavy object does no physical work because the force is not moving the object through a distance. In our everyday language, work is related to expenditure of muscular effort, but this is not the case in the language of physics. Work is done on an object when an applied force moves it through a distance. The concept of work in physics is much more narrowly defined than the common use of the word. Energy is defined as the ability to do work. The concepts of work and energy are closely tied to the concept of force because an applied force can do work on an object and cause a change in energy.
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