Work, Energy and Power-1 Flashcards
define work
The work done by the force is defined to be
the product of component of the force in the
direction of the displacement and the
magnitude of this displacement. Thus
W = (F cos θ )d = F.d
when a force acts on body displaces it, work is said tobe done. it is scalar product of force and displacement.
in which cases there is no work done
(i) the displacement is zero as seen in the
example above. A weightlifter holding a 150
kg mass steadily on his shoulder for 30 s
does no work on the load during this time.
(ii) the force is zero. A block moving on a smooth
horizontal table is not acted upon by a
horizontal force (since there is no friction), but
may undergo a large displacement.
(iii) the force and displacement are mutually
perpendicular. This is so since, for θ = π/2 rad
(= 90o
), cos (π/2) = 0. For the block moving on
a smooth horizontal table, the gravitational
force mg does no work since it acts at right
angles to the displacement. If we assume that
the moon’s orbits around the earth is
perfectly circular then the earth’s
gravitational force does no work. The moon’s
instantaneous displacement is tangential
while the earth’s force is radially inwards and
θ = π/2.
work done by tension in simple pendulum
what are two types of poisitve and negative work
Work can be both positive and negative. If θ is
between 0o
and 90o
, cos θ in Eq. (6.4) is positive.
If θ is between 90o
and 180o
, cos θ is negative.
In many examples the frictional force opposes
displacement and θ = 180o
. Then the work done
by friction is negative (cos 180o
= –1).
what is kinetic energy
Kinetic energy is a scalar quantity. The kinetic
energy of an object is a measure of the work an object can do by the virtue of its motion.
state work energy theorem
The change in
kinetic energy of a particle is equal to the
work done on it by the net force.
what is law of conservation of energy
energy can neither be reated nor destroyed only transformed from oneto another
what is spring constant
The constant k is called the spring constant. Its
unit is N m-1. The spring is said to be stiff if k is
large and soft if k is small.Due to the
opposing signs of Fs
and x, this work done is
negative,
It is the restoring force per unit displacement
what is hooke’s law
force is directly proportional to displacement in an elastic body.
does work energy theorem apply to newton’s second law
ce.
While the WE theorem is useful in a variety of
problems, it does not, in general, incorporate the
complete dynamical information of Newton’s
second law. It is an integral form of Newton’s
second law. Newton’s second law is a relation
between acceleration and force at any instant of
time. Work-energy theorem involves an integral
over an interval of time. In this sense, the temporal
(time) information contained in the statement of
Newton’s second law is ‘integrated over’ and is not available explicitly. Another observation is that
Newton’s second law for two or three dimensions
is in vector form whereas the work-energy
theorem is in scalar form. In the scalar form,
information with respect to directions contained
in Newton’s second law is not present.
what is potential energy?
potential energy is the ‘stored energy’ by virtue
of the position or configuration of a body.notion of potential energy is
applicable only to the class of forces where work
done against the force gets ‘stored up’ as energy.
conservation of mechanical energy
The total mechanical energy of a system is
conserved if the forces, doing work on it, are
conservative.
differentiate between conservative and non conservative force:
Conservative force:
- work done by the force in moving a body from one point to another does not depend on yhe path taken
-the total work done by a force through a round trip in a closed path is 0.
ex: kinetic energy, potential energy
Non Conservative force:
-work done by a force in moving a body from one point to another is path dependent.
- the work done is done by a force in a round trip is not 0
- ex: friction, viscous force,
how do we set the 0 of potential energy
The zero of the potential energy is arbitrary.
It is set according to convenience. For the
spring force we took V(x) = 0, at x = 0, i.e. the
unstretched spring had zero potential
energy. For the constant gravitational force
mg, we took V = 0 on the earth’s surface
what is power?
Power is defined as the
time rate at which work is done or energy is
transferred.
What are collisions
A collision is a short tile event in which 2 bodies interact and affect each others motion
a collision is a short term interaction between 2 or more bodies that result a change in their motion.
