Revision: Energy Physics ICSE ICSE Class 6 CISCE

“Capacity of doing work” is called Energy.

Energy, in physics, is the capacity for doing work. It may exist in potential, kinetic, thermal, electrical, chemical, nuclear, or other various forms. There are, moreover, heat and work-i.e., energy in the process of transfer from one body to another.

The work done by a force on a body is equal to the product of the force applied and the distance moved by the body in the direction of force i.e.,

Work done = Force × distance moved in the direction of force

Work is said to be done only when the force applied on a body makes the body move (i.e., there is a displacement of the body). 

The SI unit of work is joule.
1 joule of work is said to be done when a force of 1 newton displaces a body through 1 metre in its own direction.

When a force acts on a rigid body which is free to move, the body starts moving in a straight line in the direction of the force. This is called translational motion.

The energy of a body is its capacity to do work.

The ratio of the work done by the machine to the work done on the machine is called the efficiency of a machine

Efficiency =`"Output energy"/" Input energy"`

(Work done by a machine is called the output energy and the work done on a machine is called the input energy.)

The point at which energy is supplied to a machine by applying effort is called the effort point.

The point where energy is obtained by overcoming the load is called the load point.

The ratio of the velocity of effort to the velocity of the load is called the velocity ratio of the machine.

It is also defined as the ratio of the displacement of effort to the displacement of the load.

 Velocity Ratio (V.R.) =`"d"_"E"/"d"_"L"`

 Simple machine: A machine is a device by which we can either overcome a large resistive force at some point by applying a small force at a convenient point and in a desired direction or by which we can obtain a gain in speed.

Mechanical advantage (M.A.): The ratio of the load to the effort is called the mechanical advantage of the machine.

The ratio between load and effort is called mechanical advantage (M.A.).

Mechanical Advantage (M.A.) =`("Load(L)")/("Effort(E)")`

It has no unit.

 Efficiency: Efficiency of a machine is the ratio of the useful work done by the machine to the work put into the machine by the effort. In other words, it is the ratio of the work output to the work input.

Work input is work done on a machine equal to the effort force times the distance through which the force is applied.
Work output is work that is done by a machine and equals resistance force times the distance through which the force is applied.
For an ideal machine, the work output is equal to the work input, i.e., the efficiency.

 Lever: A lever is a rigid, straight or bent bar which is capable of turning about a fixed axis.

A lever is a rigid, straight (or bent) bar which is capable of turning about a fixed axis.

Input Energy = Effort × Displacement of Effort

Output Energy = Load × Displacement of Load

• The S.I. unit of work is joule (J).
    1 joule = 1 newton × 1 metre, i.e., work done when a force of 1 N moves a body 1 m in its direction.
• The C.G.S. unit of work is erg.
    1 erg = 1 dyne × 1 cm, i.e., work done when a force of 1 dyne moves a body 1 cm in its direction.
• The relation between joule and erg is:
    1 joule = 10⁷ erg

• Energy and work are directly related — when work is done, energy is transferred; doing work decreases energy, and receiving work increases it.
• No energy transfer occurs when the applied force is perpendicular to the displacement (e.g., centripetal force in circular motion).
• Units of energy are the same as those of work:
    - S.I. unit: Joule (J)
    - C.G.S. unit: Erg
    - 1 J = 10⁷ erg
• Practical units of energy:
    - 1 Wh = 3600 J = 3.6 kJ
    - 1 kWh = 3.6 × 10⁶ J = 3.6 MJ
    - 1 calorie = 4.18 J, 1 kilocalorie = 4180 J
• Energy in atomic-scale processes is measured in electron volt (eV), where 1 eV = 1.6 × 10⁻¹⁹ J

Functions and Uses of Simple Machines:

• In lifting a heavy load by applying less effort, i.e., as a force multiplier.
• In changing the point of application of effort to a convenient point.
• In changing the direction of effort to a convenient direction. 
• For obtaining a gain in speed (i.e., a greater movement of load by a smaller movement of effort).

• Output Energy = Input Energy

• Ideal Machine:
  An ideal machine is one in which no energy is lost in any manner. Here, the work output equals the work input, i.e., the efficiency of an ideal machine is 100%.

• Actual Machine:
  In an actual machine, the output energy is always less than the input energy, i.e., there is always some loss of energy during its operation.

• Reasons for Energy Loss in an Actual Machine:
  The loss of energy in a machine is due to the following three reasons:
  (1) the moving parts in it are neither weightless nor smooth (or frictionless),
  (2) the string in it (if any) is not perfectly elastic, and
  (3) Its different parts are not perfectly rigid.

• The mechanical advantage of a lever is equal to the ratio of the length of its effort arm to the length of its load arm.
  or
  \[{\mathrm{M.A.}=\frac{\text{Effort arm FA}}{\text{Load arm FB}}}\]
• The mechanical advantage of a lever can be increased either by increasing its effort arm or by decreasing its load arm.

• For class I levers, the mechanical advantage and velocity ratio can have any value, either greater than 1, equal to 1, or less than 1.
• The mechanical advantage and velocity ratio of class II levers are always more than 1.
• The mechanical advantage and velocity ratio of class III levers are always less than 1.

• It is a metallic (or wooden) disc with a grooved rim.
• A string or rope is passed around the groove at the rim. The disc rotates about an axle passing through its centre. The axle is fixed rigidly to a frame by means of nails.
• A single pulley can be used in two ways:
  (1) a fixed pulley
  (2) a movable pulley