This document discusses concepts in engineering mechanics including derived laws of forces, units of measurement, and systems of forces. It defines the triangle law and polygon law of forces, which describe the resultant force of multiple concurrent forces. Fundamental and derived units used in mechanics like the MKS and SI systems are also outlined. Finally, different types of force systems are defined, such as collinear, coplanar, and non-coplanar forces that can be concurrent, parallel or non-parallel. Idealizations commonly made in mechanics like treating bodies as rigid or applying point forces are also mentioned.
4. Triangle Law of Forces
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It is derived from the parallelogram law of forces
Triangle law of forces is
states that “if two forces
are acting on a body are
represented one after
another by the sides of the
triangle , their resultant is
represented by the closing
side of triangle taken from
first point to the last
point”
5. Polygon law of forces
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States that “if number of concurrent forces acting
simultaneously on a body are represented in
magnitude and direction by the sides of the
polygon, taken in order, then the resultant is
represented in magnitude and direction by the
closing side of the polygon , taken from first point
to last point”
7. Units in Mechanics
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Fundamental
Units
Length(L)
Mass(M)
Time(S)
Derived Units:
Sometimes, the units are also expressed in other
units( which are derived from fundamental units)
known as derived units ex: units of area, velocity,
acceleration, pressure etc
8. Units in Mechanics (Cont…)
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SystemofUnits
Meter-Kilogramme-Second (MKS) System
Centimetre- Gramme –Second (CGS) System
Foot- Pound- Second (FPS) System
9. Units in Mechanics (Cont…)
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Some quantities are expressed in MKS units as:
10. Units in Mechanics(Cont…)
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Another type of system of unit is SI( International
System) units
SI:- Meter-Kilogramme – Second
The difference between MKS and SI is mainly in
selecting the unit of force
11. Units in Mechanics(Cont…)
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Force ∝ 𝑀𝑎𝑠𝑠 𝑋 𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
Force = k X Mass X Acceleration
In SI units: a=1 m /𝒔𝒆𝒄 𝟐
Force units are: kg-m/𝑠𝑒𝑐2
is called “Newton”
In MKS units: a=g = 9.81 m /𝒔𝒆𝒄 𝟐
Force in MKS units: kg-wt is called kilogramme
weight
1kg-wt = 9.81 Newton
14. Collinear Forces
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Collinear forces are forces which have
a common line of action, i.e. the line
of action of the forces lie along a
single straight line.
22. Idealisations in Mechanics
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Assumptions in
Mechanics
The body is
rigid
The body
can be
treated as
continuum
It may be
treated as a
particleApplied force
can be
treated as
point force
Support
conditions
are idealized