5.1 Work
Work is done only when a constant force applied on an object,
causes the object to move in the same direction as the force
applied.
Work Done:
● D: the product of the component of the force parallel to the displacement times the displacement of a body.
● D: the scalar (dot) product between force and displacement of a body.
● It is a scalar quantity
● S.I. Unit is joule (J) / Kg m2 s-2
If more than 1 force acts on an object, the total work ( or net
.work ) is the sum of the work done by each force separately
or
Applications of work’s equation
• Work done, W depends on the direction of
force,F and displacement,s too.
• Examples as below:
EXAMPLE 5.1.1
You push your physics reference book 1.50 m along a horizontal
table with a horizontal force of 5.00 N. The frictional force is 1.60
N. Calculate
(a) the work done by the 5.00 N force,
(b)the work done by the frictional force,
(c) the total work done on the book.
Work Done By a
Variable Force
W = Area under ( force - displacement graph )
EXAMPLE 5.1.2
A horizontal force F is applied to a 2.0 kg radio-controlled car as it moves
along a straight track. The force varies with the displacement of the car as
shown in figure. Calculate the work done by the force F when the
car moves from 0 to 7 m.
5.3Power
Power tells you how fast work is being done or how fast energy is
transferred.
Average power, Pav
• D: the rate at which work is done.
• D: the rate at which energy is transferred
If an amount of work, W is done in an amount of time t by a force, the average power, Pav due
to force during that time interval is
• average amount of work done or energy converted per unit of time
• scalar quantity
• S.I. Unit = Kgm2s-3 / Js-1 / watt (w)
• Unit conversion between watt (W) and horsepower (hp)
Instantaneous power, P
Consider an object that is moving at a constant velocity v along a frictionless
horizontal surface and is acted by a constant force, F directed at angle above the
horizontal as shown in figure. The object undergoes a displacement of ds.
Or
EXAMPLE 5.3.1
A 70 kg person runs up a staircase 3.0 m high in 3.5 s. How
much power does he develop in climbing the steps?