Work and energy
Kinetic energy is the energy of an object in motion. The expression for kinetic energy can be derived from the definition for work and from kinematic relationships. The units are the same as for work (i.e. Joules, J). Relation bewteen KE and W: The work done on an object by a net force equals the change in kinetic energy of . Meaning of Theta Substituting the expression for work into the power equation yields P There is a relationship between work and total mechanical energy.
Work and Energy
A mass is moving and can do work when it hits another object. Electrons can flow out of a battery or capacitor and do work on another electrical component such as a light bulb.
Although massless, a photon does have energy; in the amount hf where f is the photon's frequency and h is Planck's constant. This is the energy that warms your face in the morning sun and burns your unguarded nose at the beach.
When some kinds of molecules are combined with others, energy can be released, usually as heat, light, or motion. When coal is burned it releases photon energy stored by plants millions of years before.
- Work, Power, and Energy
- What are energy and work?
- Potential energy
When hydrogen combines with oxygen to form water, heat is released as well. A fire is oxygen combining with other substances; this also produces heat.
An object must be moving to possess kinetic energy. The amount of kinetic energy KE possessed by a moving object is dependent upon mass and speed. The total mechanical energy possessed by an object is the sum of its kinetic and potential energies. Work-Energy Connection There is a relationship between work and total mechanical energy.
The final amount of total mechanical energy TMEf possessed by the system is equivalent to the initial amount of energy TMEi plus the work done by these non-conservative forces Wnc. The mechanical energy possessed by a system is the sum of the kinetic energy and the potential energy.
Positive work is done on a system when the force doing the work acts in the direction of the motion of the object.
Mechanics: Work, Energy and Power
Negative work is done when the force doing the work opposes the motion of the object. When a positive value for work is substituted into the work-energy equation above, the final amount of energy will be greater than the initial amount of energy; the system is said to have gained mechanical energy.
When a negative value for work is substituted into the work-energy equation above, the final amount of energy will be less than the initial amount of energy; the system is said to have lost mechanical energy. There are occasions in which the only forces doing work are conservative forces sometimes referred to as internal forces. Typically, such conservative forces include gravitational forces, elastic or spring forces, electrical forces and magnetic forces.
When the only forces doing work are conservative forces, then the Wnc term in the equation above is zero. In such instances, the system is said to have conserved its mechanical energy. The proper approach to work-energy problem involves carefully reading the problem description and substituting values from it into the work-energy equation listed above.
Inferences about certain terms will have to be made based on a conceptual understanding of kinetic and potential energy. For instance, if the object is initially on the ground, then it can be inferred that the PEi is 0 and that term can be canceled from the work-energy equation. In other instances, the height of the object is the same in the initial state as in the final state, so the PEi and the PEf terms are the same.
Potential energy Potential energy, also referred to as stored energy, is the ability of a system to do work due to its position or internal structure. Examples are energy stored in a pile driver at the top of its path or energy stored in a coiled spring.
Potential energy - Wikipedia
Potential energy is measured in units of joules. Gravitational potential energy is energy of position. First, consider gravitational potential energy near the surface of the earth where the acceleration due to gravity g is approximately constant. In this case, an object's gravitational potential energy with respect to some reference level is P.
Work (physics) - Wikipedia
To lift an object slowly, a force equal to its weight mg is applied through a height h. The work accomplished is equal to the change in potential energy: Launching a rocket into space requires work to separate the mass of the earth and the rocket to overcome the gravitational force. For large distances from the center of the earth, the above equation is inadequate because g is not constant.
The general form of gravitational potential energy is P. The negative sign is a result of selecting the zero reference at r equal to infinity, that is, at very large separation. Elastic potential energy is energy stored in a spring. The spring constant is a measure of the stiffness of the spring, with stiffer springs having larger k values. The potential energy stored in a spring is given by P.
Change in potential energy is equal to work. It states that energy is neither created nor destroyed but can only be transformed from one form to another in an isolated system.
Because the total energy of the system always remains constant, the law of conservation of energy is a useful tool for analyzing a physical situation where energy is changing form. Imagine a swinging pendulum with negligible frictional forces. At the top of its rise, all the energy is gravitational potential energy due to height above the stationary position.
At the bottom of the swing, all the energy has been transformed into kinetic energy of motion.