Hg
Name: Class 9
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The Hg Classes (8
th
to 12
th
) By: Er Hershit Goyal (B.Tech. IIT BHU), 134-SF, Woodstock Floors, Nirvana Country, Sector 50, GURUGRAM +91 9599697178.
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Force and Laws of Motion
Force:
• Sum of all forces acting on a body at rest or in uniform motion is zero. Such forces are called
balanced forces.
• An unbalanced force causes a change in an objects speed or its direction.
• Force of friction always opposes the motion of objects.
Newton’s Laws of Motion:
• 1
st
Law: An object at rest or in uniform motion will continue to be in the same state unless an
unbalanced force acts on it.
• Inertia: The natural tendency of an object to resist a change in their state of rest or of uniform
motion is called Inertia. Based on objects’ state it can be of 3 types -
o Inertia of rest - An object at rest has inertia of rest.
o Inertia of motion – An object in motion has inertia of motion.
o Inertia of direction – An object moving in a particular direction has inertia of direction.
Objects falling when brakes of a car are applied, or passengers jerking backwards as a train
suddenly moves forward, leaves of a tree falling when shaken, we experience an outward
force as car takes a turn can all be explained using inertia.
o The mass of an object is the measure of its inertia. Its SI unit is kg.
• Momentum: It is defined as the product of mass and velocity of a body.
o It is denoted by the letter p. p = mv
o It is also a vector quantity, its direction being the same as velocity.
o Its unit is kg m s
-1
• 2
nd
Law: The rate of change of momentum of a body is directly proportional to the force applied.
i.e., F = m(v – u)/t or F = ma [since a = (v-u)/t]
o SI unit of force is kg m s
-2
or Newton (N).
o 1 Newton is defined as the force that produces an acceleration of 1ms
-2
in an object of mass 1
kg.
• 3
rd
Law: To every action, there is an equal and opposite reaction. The actions and reaction forces
act on different bodies.
• Law of conservation of momentum: In an isolated system, (where there is no external force), the
total momentum remains conserved.
