Download Ch 6 Work, Power, Energy

Work Notes
Ch 6 Work, Power, Energy
What is Force?
Force = a push or pull on an object by
another object.
 Forces cause changes in motion

◦ Cause acceleration/ Change in direction/ Start or
Stop
SI unit of force = Newton (N)
 Force is a vector

◦ Vectors need both magnitude and direction
◦ Use arrow to indicate direction of force
The 9 Common Forces
Gravity
Friction
Applied
Normal
Spring
Magnetic
NET
Tension
Buoyant
Electrical
Gravitational Force, Fg

Symbol: Fg

Definition: Pull between 2 objects with mass
◦ gravitational attraction between an object and
earth (Weight)
Fg=m g
g = 9.8
Direction: Pull towards center of earth
 Ex. A roll of tape rests on a table

◦ gravity pulls down on roll of tape
Fg
Mass vs Weight
MASS
WEIGHT

Always remains constant

Depends on gravity

Does not depend on gravity

Weight(N)=mass(kg) x gravity(m/s2)

Unit = kg

Changes if gravity changes (ex. You
are on the moon)

Use an electronic balance to
measure


Unit = N
Eureka http://www.youtube.com/watch?v=xfGll1REu2A

Work
◦ transfer of energy through motion
◦ force exerted through a distance
Distance must be in direction of force!
W = Fd
W:
F:
d:
work (J)
force (N)
distance (m)
1 J = 1 N·m
One Joule = amount of work needed to move any
mass 1 meter with force of 1 Newton
#1 Example Problem
Brett’s backpack weighs 30 N. How much work
is done on the backpack when he lifts it 1.5 m
from the floor to his back?
GIVEN:
F = 30 N
d = 1.5 m
W=?
WORK:
W = F·d
W = (30 N)(1.5 m)
W = 45 J
W
F d
What is work????
continued

2 things are needed for there to be work
◦ Application of force
◦ Movement of object by force
 Forces parallel to direction of motion do work
 Forces perpendicular to direction of motion do no
work

Work is done when a force “helps or hinders” the motion of
an object

No motion or displacement means no work done. If
displacement = 0, work = 0
#2 Example Problem
A dancer lifts a 40 kg ballerina 1.4 m in the air and
then walks forward 2.2 m. How much work is done
on the ballerina during and after the lift?
GIVEN:
m = 40 kg
d = 1.4 m - during
d = 2.2 m - after
W=?
W
F d
WORK:
W = F·d
F = m·a
F =(40kg)(9.8m/s2)=392 N
W = (392 N)(1.4 m)
W = 548.8 J during lift
No work after lift. “d” is not in
the direction of the force.
#3 Example Problems

A teacher applies a force to a wall and becomes
exhausted.
◦ No. The wall is not displaced.

A book falls off a table and free falls to the ground.
◦ Yes. A force (gravity) acts on the book and causes book to be
displaced

A rocket accelerates through space.
◦ Yes. There is a force (the expelled gases push on the rocket)
which causes the rocket to be displaced through space.
#3 Example Problems – cont.

A waiter carries a tray full of meals above his head by
one arm straight across the room at constant speed.
◦ No. There is a force (the waiter pushes up on the tray) and
there is a displacement (the tray is moved horizontally across
the room).Yet the force does not cause the displacement.
Recall…
• Forces parallel to direction of motion do work
• Forces perpendicular to direction of motion do no work
• a vertical force does not do work
on a horizontally displaced object.
Power
Power is the rate at which work is done
 Fast or slow
 Equation

Power = work / change in time
P=W/Δt
•Unit is watt (W) or 1J/s
•Or horsepower (hp) 1hp=746W

P =W = F d = m g d = F v
Δt Δt
Δt
#5 Example Problem

If two people mow two lawns of equal
size and one does the job in half the time,
who did more work?

Same work. Different power exerted.
#6 Example Problem

A 60W light bulb does Work to produce
light and heat energy (electric energy that is
transferred into light energy). If a 60W light
bulb is on for 30s, how much work is done?
P= W/ Δt
 60W=W/30s
 1800J=W

Work-Energy Relationship
Ex. A sled, from rest, is kicked across a
frozen pond.
Amount of work done on sled = change in energy
of sled.
◦
Wnet = Δ E
◦ Net work = change in energy

Wfriction = ΔME
Video

Eureka – Work

http://www.youtube.com/watch?v=xBnS23U_ao4