7TH GRADE · WORK, POWER AND ENERGY
Efficiency and mechanical energy
Useful and total work, kinetic and potential energy.
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Basic Concepts
Efficiency
The ratio of useful work to total work expended, usually expressed as a percentage.
Due to friction and deformation, the efficiency of a real mechanism is less than one hundred percent, but the energy does not disappear, but is dissipated.
Potential energy
Interaction energy depending on the relative position of bodies or the deformation of an elastic body.
A lifted load can do work when lowered, and a compressed spring can do work when returning to its original shape.
Kinetic energy
The energy of motion of a body, which increases with mass and especially rapidly with increasing speed.
During free fall, potential energy transforms into kinetic energy if air resistance can be neglected.
Physical quantities
Efficiency
The proportion of total work converted into useful work.
Unit: dimensionless quantity
Useful work
The portion of the work expended that corresponds to the purpose of the mechanism.
Unit: joule ·
Full work
All the work that goes into running a real mechanism.
Unit: joule ·
Potential energy
The energy of a body associated with its position in a gravitational field.
Unit: joule ·
Kinetic energy
The energy of a body associated with its mechanical movement.
Unit: joule ·
Height
The vertical distance from the body to the selected zero level.
Unit: meter ·
Lesson formulas
Mechanism efficiency
Efficiency is equal to the ratio of useful work to all work expended.
Potential energy of the Earth
The potential energy of a lifted body is equal to the product of mass, gravitational acceleration and height.
Kinetic energy
Kinetic energy is equal to half the product of mass and the square of velocity.
Related formulas
The same formulas in the reference, with symbols and units.
Tasks on the topic
Statements, answer options and the walkthrough.