Ok Go!
Work,
by definition, is
the product of the force exerted on
an object and the displacement the object
moves in the direction of the force.
W = F·d
Work is a scalar quantity BUT can be
negative or positive depending on the direction
of the applied force
Graphically, work is
the area under a
“Force vs. Displacement” graph.
displacement, m
If the force and displacement are not
in the exact same direction, then
work = F(cosq)d
where q is the angle between the force
direction and displacement direction.
F =40 N
35
d = 3.0 m
The work done in moving the carrier 3.0 m
to the right by the 40 N force at an angle
of 35 to the horizontal is ...
W = F (cos q) d = (40N)(cos 35)(3.0 m) = 98 N-m
Objects that are at rest may
have many forces acting on them,
but no work is done ON THE OBJECT
if there is no movement.
Work is only done by a force on an
object if the force causes the object
to move IN THE DIRECTION of the
force.
Work done against GRAVITY
AND FRICTION!
The SI unit of work
is the Joule,
named in honor of
James Prescott Joule.
One Joule, J, of work
is the work done when
1.0 N of force is applied
through a distance of 1.0 m.
What is work done by me on luggage (10 N) to lift luggage 0.5 m?
W=Fd….(10 N)(0.5 m)…..5 J
What is work done by me on luggage (10 N) to lower luggage 0.5 m?
W=Fd….(10 N)(-0.5 m)….. -5 J
*Force opposite displacement (180 degrees)
*Cosine 180 degrees= -1
What is the net work or total work done by me on luggage?
(*Net work is the sum of all works.)
W(net)=W(up) + W(down)…..5 J + (-5 J)= 0 J
(Examples….)
Homework 5-1
MC #1-5
CQ #1-3
E # 1-4, 6, 9, 10
*In 1678, Robert
Hooke realized
that most work
that is done
involves forces
that change over
time and came up
with a relationship
to explain how a
changing force
effects the amount
of work being
done.
This relationship is known as
HOOKE’S LAW.
F ∝ x .....slope =F/x
OR k=F/x OR F=kx
k= spring constant (N/m)
The stiffer the
spring, the higher
the k constant
Hooke’s Law
Fs= -kx
Hooke’s Law
Fs= -kx
*True for “ideal
springs” but invalid
when ELASTIC
LIMIT of spring is
reached
Since Work =area under curve…
HW #5-2