Newton"s Third Law of Motion If the dynamics of a system is described
by a differential equation (or a system of
Every action have equal and opposite differential equations), then equilibria can
reaction. be estimated by setting a derivative (all
The third law states that all forces exist in derivatives) to zero.
pairs: if one object A exerts a force FA on
a second object B, then B simultaneously There are three states of equilibrium:
exerts a force FB on A, and the two forces
are equal and opposite: FA = −FB 1. Neutral equilibrium means that, with a
The third law can be used to explain the small deviation, the body remains in
generation of lift by a wing and the equilibrium. An example is a wheel rolling
production of thrust by a jet engine. on a horizontal surface. If you stop it at
any point, the wheel will be in a state of
Momentum (p) equilibrium. A ball lying on a flat
horizontal surface is in a state of neutral
It refers to the quantity of motion that an equilibrium.
object has. It can be also defined as "mass 2. Unstable equilibrium means that, with a
in motion." small deviation of the body from the
Linear momentum or translational equilibrium state, forces emerge which
momentum is the product of the mass and tend to increase this deviation. A ball
velocity of an object. located at the top of a spherical projection
It is a vector quantity. is an example of unstable equilibrium.
3. Stable equilibrium means that, with small
p mv
deviations of the body from this state,
Where p = linear momentum, m = mass, v forces or moments of forces emerge which
= velocity tend to return the body to the state of
equilibrium. A ball located at the bottom
Impulse (I) or (J) of a spherical deepening is in a state of
stable equilibrium.
It is defined as the integral of a force with
respect to time, which gives you the Friction (μ)
change in the momentum of the body
being acted on by the force. It is the force between surfaces in contact
Its unit is newton second (Ns) that resists their relative tangential motion
The quantity of impulse is force × time (slipping).
interval. The force of friction is a force that resists
motion when two objects are in contact.
Inertia Friction is high for dry and rough surfaces
and low for smooth and wet surfaces.
It is the resistance of any physical object Rolling Friction occurs when an object
to any change in its state of motion (this rolls over another (something with wheels
includes changes to its speed, direction or or that is circular like a ball). e.g. riding a
state of rest). motorcycle
It is the tendency of objects to keep Fluid Friction occurs when an object
moving in a straight line at constant moves through a fluid, meaning either a
velocity. liquid or gas. e.g. skydiving, swimming
Sliding Friction occurs when solid
Equilibrium surfaces slide over each other. e.g. falling
on the pavement
It is the condition of a system when Lubricated friction is a case of fluid
neither its state of motion nor its internal friction where a fluid separates two solid
energy state tends to change with time. surfaces.
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