ENERGY
UNIT 5
ENERGY
• What is energy?
• An object contains energy if it has the
ability to do work.
WORK
• When a force F is applied on an object and
the object moves by a distance s, we say
that F does work given by
•W=Fs
• Where:
• F= The magnitude of the applied force (N)
• s= Distance by which the object moved (m)
• W= Work in units of Nm
• Definition
• The SI unit of work is called Joule (J)
• 1J=1N ּ 1m
Meaning of Work
• The work of a force is equal to the amount
of energy transformed or transferred.
• Note:
• Work is a scalar quantity, therefore it has no
direction.
• However, if the force is in the same direction
as displacement the work is positive, while if
they are in opposite directions the work is
negative.
• The formula W=Fs is valid only if
F=constant. Otherwise, the work can only
be calculated from a graph.
• The formula given previously assumes that
the force F is parallel to the displacement
(distance) s. If this is not the case, the
formula needs to be modified:
• W=F s cosx
• where cosx=(cosine of the angle x between
the force F and the displacement s)
Examples
• A tug boat (mass of 100,000kg) pulls a
ship over a distance of 1000m in 500s. If
the pulling force (tension) is F=10,000N,
find its work.
F
• Solution
• F=10,000N
• s=1000m
• Everything else is irrelevant!
• W=Fs=10,000×1000=107J
• A limo moves on a rough road
(friction=1000N) over a distance of 100m
under the influence of a constant motive
force that does 150,000J of work. Find the
magnitude of the motive force.
s=100m
F=?
f=1000N
F=?
f=1000N
• Simply:
• W=Fs Ù F=W/s=150000/100=1500N
• Note that the presence of friction is
irrelevant!
• A limo moves on a rough road (f=1000N)
over a distance of 100m under the influence
of a constant motive force F=1400N. Find:
• (a) The work of the motive force
• (b) The work of friction
• (c) The net (total) work
• (d) What is the meaning of your answer to question (c)?
s=100m
F=1400N
f=1000N
F=1400N
f=1000N
• Solution
• (a) (Work of F)=Fs=1400×100=140,000J
• (b) (Work of