Revision: Simple Machines General Science SSC (English Medium) 6th Standard Maharashtra State Board

 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.

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"`

 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.

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).

• 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