Chapter 7 WORK, ENERGY, AND Power Work Done by a Constant Force Kinetic Energy and the Work-Energy Theorem Work Done by a Variable Force Power Examples of work. (a) The work done by the force F on this lawn mower is Fd cos θ. Note that F cos θ is the component of the force in the direction of motion. (b) A person holding a briefcase does no work on it, because there is no motion. No energy is transferred to or from the briefcase. (c) (d) (e) The person moving the briefcase horizontally at a constant speed does no work on it, and transfers no energy to it. 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. Energy is transferred to the briefcase and could in turn be used to do work. When the briefcase is lowered, energy is transferred out of the briefcase and into an electric generator. 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.
(a) A graph of F cos θ vs. d, when F cos θ is constant. The area under the curve represents the work done by the force. (b) A graph of F cos θ vs. d in which the force varies. The work done for each interval is the area of each strip; thus, the total area under the curve equals the total work done.
The speed of a roller coaster increases as gravity pulls it downhill and is greatest at its lowest point. Viewed in terms of energy, the roller-coaster-earth system s gravitational potential energy is converted to kinetic energy. If work done by friction is negligible, all ΔPE g is converted to KE.
(a)an undeformed spring has no PEs stored in it. (b)the force needed to stretch (or compress) the spring a distance x has a magnitude F = kx, and the work done to stretch (or compress) it is 1 2 kx2 (c)because the force is conservative, this work is stored as potential energy (PEs) in the spring, and it can be fully recovered. (d)a graph of F vs. x has a slope of k, and the area under the graph is 1 kx2. Thus the work done or potential energy stored is 2 1 kx2. 2
A toy car is pushed by a compressed spring and coasts up a slope. Assuming negligible friction, the potential energy in the spring is first completely converted to kinetic energy, and then to a combination of kinetic and gravitational potential energy as the car rises. The details of the path are unimportant because all forces are conservative the car would have the same final speed if it took the alternate path shown.
Work Done by a Constant Force The definition of work, when the force is parallel to the displacement: SI unit: newton-meter (N m) = joule, J
Work Done by a Constant Force If the force is at an angle to the displacement:
Work Done by a Constant Force The work can also be written as the dot product of the force and the displacement:
The work done may be positive, zero, or negative, depending on the angle between the force and the displacement:
If there is more than one force acting on an object, we can find the work done by each force, and also the work done by the net force:
Kinetic Energy and the Work-Energy Theorem When positive work is done on an object, its speed increases; when negative work is done, its speed decreases. After algebraic manipulations of the equations of motion, we find: Therefore, we define the kinetic energy:
Work-Energy Theorem: The total work done on an object is equal to its change in kinetic energy. Work Done by a Variable Force If the force is constant, we can interpret the work done graphically: If the force takes on several successive constant values:
We can then approximate a continuously varying force by a succession of constant values.
The force needed to stretch a spring an amount x is F = kx. Therefore, the work done in stretching the spring is
Conservative and Nonconservative Forces Potential Energy and the Work Done by Conservative Forces Conservation of Mechanical Energy Work Done by Nonconservative Forces Potential Energy Curves and Equipotentials Conservative force: the work it does is stored in the form of energy that can be released at a later time Example of a conservative force: gravity Example of a nonconservative force: friction Also: the work done by a conservative force moving an object around a closed path is zero; this is not true for a nonconservative force
Work done by gravity on a closed path is zero: Work done by friction on a closed path is not zero:
The work done by a conservative force is zero on any closed path: The Work Done by Conservative Forces If we pick up a ball and put it on the shelf, we have done work on the ball. We can get that energy back if the ball falls back off the shelf; in the meantime, we say the energy is stored as potential energy.
Conservation of Mechanical Energy Definition of mechanical energy:
Work Done by Nonconservative Forces In the presence of nonconservative forces, the total mechanical energy is not conserved:
Potential Energy Curves and Equipotentials The curve of a hill or a roller coaster is itself essentially a plot of the gravitational potential energy:
Potential Energy Curves and Equipotentials The potential energy curve for a spring:
Power Power is a measure of the rate at which work is done: SI unit: J/s = watt, W 1 horsepower = 1 hp = 746 W If an object is moving at a constant speed in the face of friction, gravity, air resistance, and so forth, the power exerted by the driving force can be written:
Summary If the force is constant and parallel to the displacement, work is force times distance If the force is not parallel to the displacement, The total work is the work done by the net force: SI unit of work: the joule, J Total work is equal to the change in kinetic energy: