Aslevels Physics Notes
Topic 6: Work , Energy and Power
Energy
Energy is defined as the ability (or capacity) to do work. Energy is measured in joules. When a body has 300J of energy it means that it can do 300J of work.Different forms of energy are shown in the table below:
Conservation of energy
The law of conservation of energy states that the total energy of a closed system is
constant. This means that energy can be transformed from one form to another but it
can neither be created nor destroyed β the total energy of a closed system will be the
same before an interaction as after it. When energy is transformed from one form to
another either:
- Work is done β for example, a man does work against gravity by lifting a large mass onto his shoulders.
Or
- Energy is radiated or received in the form of electromagnetic radiation β for example, internal energy is radiated away from the white hot filament of a lamp by infrared and light radiation.
Gravitational potential energy
Consider a mass m is lifted through a height h.
The weight of the mass is mg, where g is the gravitational field strength.
π€πππππππ = πππππ Γ πππ π‘ππππ πππ£ππ = ππββ
Due to its new position, the body is now able to do extra work equal to ππββ. It has
gained extra potential energy, βπ = ππββ
ππππππ ππ πππππππππ ππππππ = ππβh
If the net force acting on an object in a particular situation has a constant value, or if it
is appropriate to utilize an average force value, then:
The net work done on an object is defined as the product of the net force on the object
and its displacement in the direction of the net force. When the force and displacement
are in the same direction, the work done by the stated force is given by:
π = πΉ. π
Where:
W is the work done by the stated force in joules (J)
F is the magnitude of the stated force in newtons (N)
S is the magnitude of the displacement in meters (m)
F is the magnitude of the stated force in newtons (N)
S is the magnitude of the displacement in meters (m)
Work is the area under a force β displacement graph.
Situations also occur in which a constant force acts at an angle π to the direction of
motion. A force acting at an angle will be less effective than the force acting solely in the
direction of the displacement. The component of the force in the direction of the
displacement, πΉπππ π, is used in calculating the work done in the required direction.
The efficiency of energy transformations
The percentage of energy that is transformed to a useful form by a device is known as
the efficiency of that device. All practical energy transformations βloseβ some energy as
heat. The effectiveness of a transfer from one energy form to another is expressed as:
Mechanical energy
Mechanical energy is defined as the energy that a body possesses due to its position or
motion. Kinetic energy, gravitational potential energy and elastic potential energy are all
forms of mechaical energy.
Kinetic energy
An object in motion has the ability to do work and therefore is said to possess energy.
This energy carried by a moving object is called kinetic energy (from the Greek word
βkinesisβ: literally meaning motion).
If a moving object of mass, m, and initial velocity u, experiences a constant net force, F,
for time, t, then a uniform acceleration results. The velocity will increase to a final value,
v. work will have been done during the time the force is applied.
From the definition of work, when the net applied force is in the direction of the
displacement .
π = πΉS
Now substituting Newtonβs second law πΉ = ππ we get
π = πππ
Using one of the equations of motion
2ππ = π£ Β² β π’Β²
π =
π£Β² β π’Β²/ 2π
Power
Power (P) is the rate of doing work or transforming energy
Power is measured in joules per second. One joule per seond is 1 watt. The symbol for
watt is W.
Power and velocity
Consider a car travelling at constant velocity v along a straight, level road. The engine
must continue to do work against friction. If the firctional force is F, then the engine will
supply an equal-sized force in the oposite direction. The work done by the engine, βπ, in
time βπ‘ is πΉβπ, where βπ us the distance travelled in time βπ‘.
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