Transcript Chapter 5 Notes

What is Work?
• Work: Force multiplied by distance
moved in the SAME direction as the
force.
Units: Newtons x Meters = Joules
Work vs. Force
• Work: done ON an object
• Force: what makes work happen
• WORK IS DONE BY FORCES
Calculating Work done by a pulley:
Work= Force (on rope) X Distance (block moves)
Efficiency
•The work output of a machine can NEVER
exceed the work input.
•In real life, usually you lose a lot of work due
to things like friction.
•You get a mechanical advantage because you
trade FORCE for DISTANCE.
•The efficiency of a machine is the ratio of
work output to work input.
How to Calculate a Machine’s Efficiency
Efficiency % = Work Output
Work Input
x 100
The Rate at which work is done is called Power.
The unit of power is called the watt.
POWER (watt) = Work (joules)
Time (sec)
• Horsepower is another commonly used unit
for power.
• 1 unit of Horsepower = 746 watts
• (was the average power output of a Horse)
Example
• Suppose you drag a box with a force of 100
newtons for 10 meters, and it takes you 10
seconds. Your friend drags a similar box but
takes 60 seconds.
• How much work you did you EACH perform?
• Who did more work?
• Calculate you and your friend’s power.
• Who had more Power? Why?
• Energy: the ability to do work.
• Any object that has ENERGY can create a FORCE
•Examples:
Moving ball: creates a force on
whatever tries to stop it.
Sled at TOP of hill: it CAN go
down the hill and produce forces
as it goes.
Electricity: can turn an engine
on.
Gasoline: can be burned in an
engine to make forces to move a
car
Potential & Kinetic Energy
Potential Energy: Depends on position of object relative
to Earth…. like stored energy that can be used later.
Potential means that the object/ matter COULD become
active. Objects that have POTENTIAL energy don’t
use it UNTIL THEY MOVE!!
 Potential Energy : relates object’s weight
(Newtons) with position.
 Potential Energy….It COULD happen…
Kinetic Energy: What’s Happening NOW!
Kinetic Energy: Energy of motion that depends on two
things… MASS and Velocity
A moving mass can exert forces on things… like getting
bumped into in the hall
The more mass an object has, the more KINETIC energy it
has….takes more force to move a more massive object…
More force = More work = More energy
ENERGY is measured in JOULES (J),
just like work.
Moving from Potential to Kinetic and
Back AGAIN!
• Energy can
NEVER be
CREATED or
DESTROYED,
just transformed
from one form to
another.
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EXAMPLE
The following diagram shows a cart rolling
along a hilly road. Ignore the effect of friction.
Arrange the five locations in order of increasing
potential and kinetic energy.
Types of Energy:
Can all move from one type to another!
Chemical to
Potential….
but how do we
get to Kinetic?
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OH, that’s how….
Speed up Down
the Hill, Potential
to Kinetic
Put on the
Brakes…. Kinetic
back to Potential
Define the other types of Energy using your book.
Mechanical
Radiant
Electrical
Chemical
Nuclear
Thermal