Work Power and Energy

1
Work Power and Energy

• By,
• Dr. Ajay Kumar
• School of Physical Education
• D.A.V.V. Indore

2
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.

3
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.

4
Equation for Work

• W F X d
• Where W Work
• F Force
• d Distance

5
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.

6
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.

7
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.

8
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.

9
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, F Force,
• d Distance, W Work, t Time

10
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
  Foot-Pound / 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

11
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.

12
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

13
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

14
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)

15
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

16
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

17
Work-Energy Relationship

• The work done by the net force acting on a body
  is equal to the change in the bodys kinetic
  energy
• This relationship is true as long as there is no
  change in vertical position.

18

• 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

19

• 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.