FORM 2-PHYSICS-TOPIC 5-SIMPLE MACHINES
SIMPLE MACHINES
A simple machine is a non-powered mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force. A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force.
A pulley is a grooved wheel, which is free to turn about an axle that is fixed in a frame.
Different Pulley System
Identify different pulley systems
Types of Pulleys
- Single fixed pulley
- Single movable pulley
- The block and tackle system of pulley
Single fixed pulley
This is the type of pulley whereby effort is applied at one end of the tape in order to raise the load. Single fixed pulleys are used to raise small objects e.g. Flags. Consider the diagram below:-
Single fixed Pulley
M.A and V.R of single fixed pulley
Single Movable Pulley
Single movable pulley is the one which load is multiple of Effort.
Load = 2E
Consider the following diagram
Single movable pulley
M.A and V.R of single movable pulley
The block and tackle system of pulley
When two or more pulleys are fixed in a frame, a block is formed. The pulleys in each block are fixed independently on separate axles. Consider the diagram below:-
Block and tackle system of pulley
Note; mechanical advantage = Load/Effort
Velocity ratio of Block and tackle system is equal to the number of pulleys.
Example 5
Example 1
A block and tackle pulley system has a velocity ratio of 4. If a load of 200N is raised by using a force of 75N. Calculate the mechanical (i) Advantage of the system (ii) efficiency of the system.
Solution;
Data given
Velocity ratio (V.R) = 4
Load = 200N
Effort = 75N
M.A =?
M.A and Efficiency of block and tackle pulley system
Mechanical Advantage, Velocity Ratio and Efficiency of Pulley System
Determine mechanical advantage, velocity ratio and efficiency of pulley system
VR of block and tackle
When the lower two pulleys move up a vertical distance y, corresponding to movement y of the load L, each pulley releases a length y of the rope on each side giving a total length 2y.
With all this movement of pulleys, the effort E is moved down a distance of 2 x 2y or 4y.Thus, VR = (distance moved by effort)/(distance moved by load at the same time) = 4y/y = 4
VR = Number of pulleys of the system.
Efficiency of block and tackle
From, ε = MA/VR X 100%
ε = (number of pulleys )/(number of pulleys) x 100%
Note:Then, for a perfect block and tackle pulley system, ε = 100%.
But in practical case the MA is less than the number of pulleys, hence the efficiency of pulleys system is less than 100% due to friction losses.
MA of block and tackle.
From MA = load/effort
MA = (Effort x Number of pulleys of the system)/Effort
MA = Number of pulleys of the system.
Pulley in Daily Life
Use of pulley in daily life
Uses of Pulleys
Pulleys have been used for lifting for thousands of years. The most prevalent and oldest example are their uses on ships and boats. The block and tackle have been a key tool for raising sails and cargo. Another major use for pulleys is withcranes.
Pulleys have been used also in modern times with various machines and systems. Even in the space age, pulleys have been an important aspect for the construction and operations of spacecraft and aircraft. It is with a pulley system that rudders for an aircraft are controlled.
Pulleys are used in everyday life, from vehicles to moving equipment such as cranes.
