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SCIENCE-9

Chapter 3 - Simple Machine

Introduction

A Simple Machine is a device that makes our work easier, change the direction of the force and works with more efficiently.
A simple rod, knife, scissor and a wheelbarrow are the example of simple machines. They help us to do our work easily and quickly. When we work on the machine, mechanical energy of the machine changes into force and that's the force which does the work.

Mechanical Advantage (M.A.): The ratio of load lifted by a simple machine to the effort applied on it is called mechanical advantage.

MA = Load (L) / (Effort)E


It does not have any unit since it is the ration of two forces.
A man lifted a load of 600 N using a simple machine. If he applied 200 N effort, then the mechanical advantage of this machine is (M.A) = 600 / 200 = 3.
Note: The Mechanical Advantage more than 1 multiplies Effort, less than 1 speeds the doing works, and equal to 1 makes the work easy and change the direction of effort.

Velocity Ratio (V.R.): The ratio of velocity of effort to the velocity of load is called the velocity ratio of the simple machine.

Velocity Ratio = (Velocity of effort) / (Velocity of Load)

if load and effort cover certain distance in the same time, then
V.R = (Distance covered by effort / time) / (Distance covered by load / time)

V.R = (Distance covered by effort ) / (Distance covered by load )

So, V.R = (Effort Distance) / (Load Distance)

So, Velocity ratio may also be defined as the ratio of distance covered by effort to the distance covered by load.

It also have no unit since it is the ratio of two distances.
Note: Velocity ratio of simple machine is always more then mechanical advantage because there is no effect of friction in VR but effect can be seen in MA.

Efficiency (η): The percentage ratio of output work to the input work of a simple machine is called efficiency.

Efficiency(η) = (load × Load distance) / (effort × effort distance) × 100%

Efficiency(η) =(MA) / (VR) × 100%

An ideal or perfect machine has output work equal to input work and its efficiency is 100% but, in practical liofe it is impossible, no any machine has 100% efficiency because there is always some losses of energy due to friction.

Relation Between M.A., V.R., and η :
Efficiency(η) =(MA) / (VR) × 100%

Answer: what does it mean, A machine has 70% efficiency?

Simple Machine is of Six types:

  1. Lever: A lever is a bar which can turn about on one point. The point is called a fulcrum, the force we use on the lever is called a effort, the weight we try to lift or move it is called load.

    Lever are further classified into three types:
    A) First Class Lever: In the first class lever, the fulcrum is between the load and the effort.
    Some common first class levers are see-saw, crowbar, pliers, scissors, etc

    B) Second Class Lever: In the second class lever, the fulcrum is at one end and the load lies between the effort and fulcrum.
    Some common second class levers are stapler, botle opener, lemon squeezer, etc

    C) Third Class Lever: In the third class lever, the effort lies between load and fulcrum.
    Some common third class levers are broom, our arms, etc.

  2. Pully: It is a circular disc having groove made on its circumference to pass rope or string. It is made by wood or metal or hard substances.Its uses is to lift a heavy load of greater height.
    Like: used in construction site, pulling of water from well, etc.

    Velocity ratio (V.R.) = The number of rope segments used to hold the load
    = The number of pulleys used
    In a single movable pully the Velocity Ratio(VR) is two.

  3. Wheel and Axle: It consists of two co-axial cylinders of different diameters. The cylinder having larger diameter is called wheel whereas the cylinder having smaller diameter is called axle.
    The effort is applied in wheel and they work jointly. String roller of a kite, knob of a door etc are the example of wheel and axle. It is used to lift heavy loads.

    The ratio of radius of wheel to the radius of axle is called velocity ratio of the wheel and axle.
    V.R. = (Radius of wheel R) / (Radius of axle r)

  4. Inclined Plane: A slanted surface made by a plank of wood or metal sheet is called Inclined Plane.
    Heavy loads can be pulled or pushed up by applying little effort along with it. It makes our work easier by magnifying force. a wooden plank used to load or unload truck, steep road, staircase etc are the examples of Inclined plane.
    Mechanical advantage in inclined plane is also the ratio of load lifted to the effort applied.

    M.A = Load / Effort

    The velocity ratio in the inclined plane is the ratio of length of slope to the vertical height of the inclined plane

    V.R = (Length of the slope) / (Height of the Inclined plane)
    V.R = d / h


Moment:
  • Moment of the force may be defined as the product of the force and its perpendicular distance of the line of action from the axis of rotation.
  • Mathematically,
  • Moment = f × d
  • Its unit is Nm
Laws of Moment:
    Law of moment states that, “in the equilibrium condition the sum of clockwise moments about a point must be equal to the sum of the anticlockwise moments about the same point.”

    You felt easy to apply force at the end of the spanner to unscrew the nut, why?




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