Transcript Work Power and Energy - School of Physical Education

Work Power and Energy
By,
Dr. Ajay Kumar
School of Physical Education
D.A.V.V. Indore
Work
• Machine are designed to do work.
• Simple machine such as the lever or wheel are
the devices which are designed to perform
work more efficiently.
• In each instance the machines aids in the use
of a force to overcome a resistance efficiently.
• When the resistance is overcome for a given
distance , work is done.
Definition of Work
• Mechanically speaking, “ work
is the
product of the amount of force
expended and the distance through
which the force succeeds in
overcoming a resistance it acts
upon.”
Equation for Work
W=FXd
• Where
W = Work
F = Force
d = Distance
Unit of Work
• Unit for expressing work are numerous.
• In English system the foot – pound is the
most common unit.
• Joule is the most frequently used unit in
metric system.
• A joule is equivalent to 107 x one gram of
force exerted through one centimeter.
Work (Cont)
• In computing work, the distance “d” must
always be measured in the direction the force
acts.
• Work done in the same direction that the body
moves or Concentric movement is called as
positive work.
• Work done in the opposite direction or
eccentric movement is called negative work.
Work (Cont)
• Negative forces resisting gravity perform
less work over a given distance than
positive forces overcoming gravity.
• Example: One perform more work walking
up a mountain the walking back down.
Work (Cont)
• When the exertion of effort produces no
motion, mechanically speaking no work is
done.
• The physiological measures of such efforts
may be determined by obtaining energy cost.
This is usually measured by computing the
amount of oxygen consumed during the effort
and converting it to calories per minute.
Power
• Any measures of work does not account for the
time involved in performing the work.
• The rate at which work is done is called power
and may be expressed as:
P = Fd / t or
P=W/t
Where,
P= Power,
d = Distance,
F = Force,
W = Work, t = Time
Power (Cont)
• From the equation it is clearly evident the the
machine or person who perform more work in
a given time is more powerful.
• In english system power is expressed as FootPound / Sec or Horse power
( 1 Horse power = 550 Ft-lb / sec)
• In metric system the unit is watt, which is
equivalent to one Joule / Sec
Energy
• Energy is defined as capacity to do work.
• A body is said to posses energy when it can
perform work.
• Energy may take numerous form, and can
be converted from one form to another.
• According to the Law of Conservation of
Energy it can neither be created nor
destroyed.
Energy (cont)
• Energy = the capacity to do work (scalar)
• Types of energy: mechanical, chemical, heat,
sound, light, etc
• In sports we are most interested in mechanical
energy
Mechanical Energy
• Kinetic Energy (KE) - energy due to motion
• e.g. a diver (mass = 70 kg) hits the water after a
dive from the 10 m tower with a velocity of 14
m/s. How much KE does she possess?
• KE = ½ mv2 = ½ x70 kg(-14 m/s)2 = 6860 J
Mechanical Energy
• (Gravitational) Potential Energy (P.E.)
• – energy due to the change of position in
gravitational field
PE = mgh
h = height of something above some reference line
m = mass
g = acceleration due to gravity (–9.81 m/s2)
Potential Energy:
• Note: in the absence of air resistance and other
resistive forces, PE can be completely converted
to KE by the work done by gravity on the way
down.
• e.g. a diver on top of a 10 m tower has a positive
PE compared to water level
• PE = –mgh = –(70 kg) x (–9.81 m/s2) x (+10 m)
= 6860 J
Mechanical Energy
• Strain or elastic energy (SE)
• – energy due to deformation
• – this type of energy arises in compressed
springs, squashed balls ready to rebound,
stretched tendons inside the body, and other
deformable structures SE
Work-Energy Relationship
• The work done by the net force
acting on a body is equal to the
change in the body’s kinetic energy
• This relationship is true as long as
there is no change in vertical
position.
• The kinetic energy of a body is the energy
due to its motion. The faster a body moves
the more kinetic energy it posses. When a
body stops moving the kinetic energy is
lost. This is easily seen in the equation of
kinetic energy.
•
K.E. = ½ mv²
• According to the principle of the
conservation of energy the work done is
equal to the kinetic energy acquired and
therefore
•
FD = ½ mv²
• This relationship is extremely helpful in
explaining the situation when receiving the
impetus of any moving object.