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A simple machine is a mechanical device that makes our life easier. If a force is applied at one point, the simple machine transmits it to another point with a convenient change of magnitude and direction. In this article we shall study the terminology associated with the simple machines.
Types of Machine:
The basic machines are
- Lever
- Inclined plane
- Pulley (Special case of levers)
- Wheel and axle (Special case of levers)
- Wedge (Special case of the inclined plane)
- Screw (Special case of the inclined plane)
- Gear
Functions of Simple Machine:
- Applying force at a convenient point: Instead of applying force directly to the wheels of a bicycle, it is easier and more convenient to apply it to the pedals.
- Applying force in a convenient direction: It is difficult to lift a bucket full of water directly, but the task becomes very easy if the force is applied in a downward direction using a pulley.
- By applying small effort to lift large loads: In such case machine is said to be used as a force multiplier. e.g. a screw jack used to lift a car or a truck.
- To change the speed of motion of a body: In such a case the machine is said to be used as a speed multiplier. e.g. Gears in an automobile are used to change the speed of the automobile.
Defining a Simple Machine:
Depending upon the above mentioned functions of simple machines we can define simple machine as follows: “A simple machine is a device by which we can either overcome a large resistive force (or load) at some point by applying a small force (or an effort) at convenient point and in a desired direction or by which we can obtain a gain in speed”.
Terminology:
Load (L):
A resistive force to be overcome by a machine is called a load. Its S.I. unit is newton (N)
Effort (E):
An external force applied to a simple machine to overcome a load is called an effort. Its S.I. unit is newton (N)
Load Point:
The point where the energy is obtained by overcoming the load, is called the load point.
Effort Point:
The point at which the energy is supplied to a machine by applying the effort, is called the effort point.
Mechanical Advantage (M.A.):
The ratio of load (L) to overcome to the magnitude of the effort (E) is called a mechanical advantage. It is also called as leverage.
As it is ratio of same type of physical quantities, the mechanical advantage is unit less and dimension less quantity.
- If the effort needed is less than the load, the machine has mechanical advantage greater than 1. In such case the machine acts as a force multiplier.
- If the effort needed is greater than the load, the machine has mechanical advantage less than 1. In such case the machine helps in gaining speed.
- If the effort needed is equal to the load, the machine has mechanical advantage equal to 1. Generally, in such cases the machine is used to change the direction of the effort.
Velocity Ratio:
The ratio of a distance travelled by the effort to the distance travelled by the load in given time is called velocity ratio.
As it is ratio of same type of physical quantities, the velocity ratio is unit less and dimension less quantity.
- If the distance travelled effort is more than the distance travelled by load, the machine has velocity ratio greater than 1. In such case the machine acts as a force multiplier.
- If the distance travelled effort is less than the distance travelled by load, the machine has velocity ratio less than 1. In such case the machine helps in gaining speed.
- If the distance travelled effort is equal to the distance travelled by load, the machine has velocity ratio equal to 1. Generally, in such cases the machine is used to change the direction of the effort.
Work Input:
The energy supplied to a machine is called work input. Its S.I. unit is joule (J).
Work Input = Effort (E) X Distance traveled by an effort (s)
Work Output:
The useful work done by a machine is called work output. Its S.I. unit is joule (J).
Work Output = Load (L) X Distance traveled by a load (l)
Efficiency:
The ratio of the useful work output to the actual work input of the machine is called efficiency.
For a machine, of a given design, the velocity ratio does not change, but due to friction and weight of the moving parts of machine mechanical advantage and the efficiency decrease.
Ideal Machine:
Ideal machine is that machine in which there is no loss of energy in any manner i.e. the work output is equal to the work input.
Thus, for ideal machine the mechanical advantage of the machine is equal to its velocity ratio and the efficiency of the machine is 100%. In practice there is no ideal machine because for every machine the output energy is always less than the input energy. Thus there is a loss of energy in using the machine.
Actual Machine:
Actual machine is a machine in which the output energy is always less than the input energy i.e. there is a loss of energy in using the machine.
Thus, for actual machine the efficiency is always less than 100%. Thus efficiency of machine is 80% means 20% of the input energy is lost in machine and only 80 % of input energy is converted into useful work.
The reasons for loss of energy in using the machine are as follows:
- The friction between different moving parts in the machine,
- The different parts of machine are not perfectly rigid,
- The string used in machine is not perfectly elastic,
- The different parts of machine have some weight (inertia), and
- The different parts used in machine are not smooth
Principle of a Machine:
The work output of a machine is equal to the work input. In practice work output of machine is not equal to the work input. Work output of machine is always less than the work input.
Example – 01:
In a machine an effort of 100 N is applied to lift a load of 1000 N. What is its mechanical advantage?
GivenEffort = P = 100 N, Load = W = 1000 N
To Find: Mechanical Advantage = M.A. =?
Solution:
M.A. = W/P = 1000/100 = 10
Ans: Mechanical advantage is 10.
Example 02:
A machine has mechanical advantage 5. It raises a load of 25 N. Calculate the minimum effort required.
Given: Mechanical advantage = M.A. = 5, Load = W = 25 N
To Find: Effort = P =?
Solution:
M.A. = W/P
P = W/M.A. = 25/5 = 5 N
Ans: Efforts required = 5 N
Example – 03:
The mechanical advantage of a machine is 2. It is used to raise a load of 150 N. What effort is needed?
Given: Mechanical advantage = M.A. = 2, Load = W = 150 N
To Find: Effort = P =?
Solution:
M.A. = W/P
P = W/M.A. = 150/2 = 75 N
Ans: Efforts required = 75 N
Example 04:
The mechanical advantage of a machine is 5. How much load it can exert for the effort of 20 N?
Given: Mechanical advantage = M.A. = 5, Effort = P = 20 N
To Find: Load = W =?
Solution:
M.A. = W/P
W = M.A. x P. = 5 x 20 = 100 N
Ans: Load = 100 N
Example 05:
The efficiency of a machine is 50 %. If 300 J of energy given to the machine. What is its output?
Given: Efficiency of machine = 50%, Input energy = 300 J
To Find: Output energy
Solution:
Efficiency = (Output / Input) x 100
Output = (Efficiency x Input) / 100
Output = (50 x 300)/100 = 150 J
Ans: Output of machine is 150 J
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