Work, Power, Machines
Chapter 8 Part One
Work
• In physics, work is done when a forcemoves an object through a distance.
• When you push a box up an incline, you “do work” on the box.
dFW ⋅=
Work = Force x Distance
Units
• Force is measured in Newtons.
• Distance is measured in meters.
• N x m = Nm = Joules
• Joules and Nm are the same things.
Force and Distance must be in the same direction!!
• If you pick an object up, the force must be the upward force.
• If you slide an object sideways on a table, the force must be the sideways force.
• If you measure the distance up an incline, the force must be the one exerted in a direction up the incline.
Is work vector or scalar.• Force and displacement are both vector.
• But, due to complicated math
(vector dot products), work is scalar.
• Power and energy are scalar as well.
• It does make the math easier.
– Use arithmetic instead of vector addition.
– Joules and watts don’t have direction.
How much work do you do to lift a 15 N object upward 4 meters?
• When you pick an
object up, you have to
apply a force equal to
its weight (15 N). (why?, CVPM? UAPM?)
W = F x d
W = 15 N x 4 m = 60 J
How much work do you do to lift a 5-kg object upward 4 meters?
• When you pick an
object up, you have to
apply a force equal to
its weight
(5kgx10m/s2=50 N).
W = F x d
W = 50 N x 4 m = 200 J
How much work do you do to slide a 15 N object 4
meters sideways if the force of friction is 3 N?
• When you slide an
object sideways, you
have to apply a force
equal to friction (3 N). (why? CVPM? UAPM?)
W = F x d
W = 3 N x 4 m = 12 J
How much work do you do to slide a 15 N object
up an incline 4 meters long if the force the person
must push with is 7 N?
• When you slide an
object up an incline,
you have to use the
force up the incline
(7N).
W = F x d
W = 7 N x 4 m = 28 J
How much work do you do to slide a 5-kg object
sideways 4 meters if the acceleration is 6 m/s2?
F=ma
F = 5-kg x 6 m/s2
F = 30-N
W = F x d
W = 30 N x 4 m = 120 J
Sometimes work can be negative!
• If the force and displacement are
exactly opposite, then work is negative.
• When you lift a box, you pull upward and do positive work.
• But gravity pulls downward and does negative work!
How much work is done by…
• Fa
(10)(15)= 150J
• FF
(-10)(15)= -150J
• FN
(100)(0)= 0J
• Fg
(100)(0)= 0J
dFW ⋅=
CVPM tells you what about the forces?
WHY?!
How much work is done by…
• FT
(30)(6)= 180J
• Fg
(-20)(6)= -120J
WHY?!
Special Case – No Work Done!
• The force (weight) is
downward. There is
virtually no sideways
force.
• The distance is
sideways. There is no
downward motion.
W = 14-N x 0-m = 0J
W = 0-N x 10-m = 0J
Assignment 8a
• Due Tomorrow
• Start the Pulley Lab Today