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WORK – ENERGY – POWER
The WORK done by constant force acting on a body is defined as the scalar product of the displacement and the force.
: is the component of in the direction of the displacement
: is the displacement
: is the angle between and
In the SI, the unit for work is but the unit of the work has a special name Joule :
is the work done by a force of one Newton through one meter along the line of the force.
Work is a scalar quantity
Work can be if the angle between the force and displacement is or as for these angles:
If , the work done by the applied force is positive
If , work is negative (e.g.: when an object is lifted, the work done by the gravitational force on the object is negative)
Important consideration that: work is an energy transfer.
WORK DONE BY A FORCE
Work done by a force is represented and calculated as the area under the curve of versus displacement , where is that component of the force, which is parallel to .
Efficiency:
POWER is the rate at which work is done:
The SI the unit for power is called Watt (W) which is equivalent with the Joule per second:
Note:
1) For motors and engines power is often named in horsepower, where
2) Commonly used the kilowatt:
and the megawatt:
3) A conveniently large energy unit in common usage is the kilowatt-hour (Not unit of power!):
4) If many forces are acting on an object, you can calculate the work done by each force individually. The net work will be the sum of these. Or we can find the net force and calculate the work done by it.
ENERGY
is a property of objects, the measure of the ability of the object to perform work, which can be transferred to other objects or converted into different forms, but cannot be created or destroyed.
KINETIC ENERGY: the energy possessed by a body as a result of its motion (translational or rotational )
The translational KINETIC ENERGY of an object of mass moving with a speed is:
Change in KE= Work done by
Work-energy theorem for net force
Note:
POTENTIAL ENERGY: the energy possessed by a body as a result of its position or configuration
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GRAVITATIONAL POTENTIAL E:
The object has a potential energy as a result of its position in the gravitational field of the Earth
Example: waterfall
ELASTIC POTENTIAL E:
The object has a potential energy as a result of change its elastic body
Example: the clock spring when the clock is wound
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The change in gravitational or elastic potential energy when a body changes from one position or configuration to another is the work done against the gravitational or elastic forces during the change
GRAVITATIONAL POTENTIAL ENERGY
When we are lifting up a distance an object, which has the mass with constant speed, then we need to exert a force against the gravitational force.
LIFTING WORK DONE =
The work the object can do by its position in the gravitational field is its potential energy: .
Note:
Gravitational potential energy depends on the height or vertical position of the object’s center of mass relative to some specified reference level.