Download Mechanical Energy and Work

Mechanical Energy and Work
Energy-An Agent of Change
• 
Mechanical energy - total
amount of kinetic and
potential energy in a system
– 
• 
(ME)
When an object can change
its environment, it has energy
(e.g baseball breaking
window)
– 
Energy- ability to cause change
(E)
Types of Mechanical Energy
Two types of ME:
1.  Kinetic Energy
2.  Potential Energy
Check your brain
•  Energy used to be defined as ability to
do work - to cause something to move.
Why does the new definition “the
ability to cause change” work with the
old definition?
Check your brain
•  Energy used to be defined as ability to
do work - to cause something to move.
Why does the new definition “the
ability to cause change” work with the
old definition?
•  When you move something, there is a
change so they mean the same thing.
Kinetic Energy (KE)
•  Kinetic energy- energy in the
form of motion
–  E.g. spinning bicycle wheel,
sprinting, flying Frisbee
•  Amount of KE depends on mass
and velocity of moving object
•  Velocity: Speed in a given
direction
–  Greater mass = more KE
–  Greater velocity = more KE
Check your brain
•  E.g. Does a truck traveling at same speed as
motorcycle have more or less KE?
Check your brain
•  E.g. Does a truck traveling at same speed as
motorcycle have more or less KE?
•  Truck, it has more mass, which means more
molecules are moving.
Check your brain
•  E.g. Does a motorcycle traveling 100
miles per hour have more or less KE
than an identical motorcycle traveling at
80 miles per hour?
Check your brain
•  E.g. Does a motorcycle traveling 100
miles per hour have more or less KE
than an identical motorcycle traveling at
80 miles per hour?
•  The one moving at 100 miles per hour,
if it is going faster, there is more E.
Potential Energy (PE)
•  Potential energy- stored
energy
–  Depends on its position or
condition
•  E.g flowerpot in window sill on
2nd story has PE
–  If it is knocked over, gravity
will cause it to fall towards
ground and energy turns from
PE to KE
–  Greater height = greater PE
–  Less stable = greater PE
Check your brain
•  E.g. Does a flowerpot on lower floor or
5th floor have more PE?
Check your brain
•  E.g. Does a flowerpot on lower floor or
5th floor have more PE?
•  The 5th floor, it is higher so there would
be more of a change if it were to fall
than the flower pot that was already on
the first floor.
–  Gravitational potential energyanything that has a mass has
GPE because of its height from
the earth
•  E of falling down
Gravitational
What is work?
• 
Work- transfer of energy
through motion
– 
– 
• 
In order to take place, force
must be exerted
Force - push or pull
Depend on 2 things:
1.  Amount of force exerted
2.  Distance force is applied
• 
Calculated by:
Work = Force x Distance
(W= F x d)
How can you tell work is being done?
• 
Two factors
1.  Something has to move
2.  The motion must be in the same
direction as the applied force
• 
E.g Work = Pick up pile of
books on floor
– 
• 
They move upward, in direction
of applied force
E.g. Not work = Hold books in
arms
– 
Some upward force being
applied (so books don’t fall), but
no movement
Check your brain
• 
If you are carrying books at constant speed
across floor, are you doing work on the
books?
Check your brain
• 
If you are carrying books at constant speed
across floor, are you doing work on the
books?
•  You are not doing work on the books.
Your force on them is upward. Their
motion is horizontal
Conservation of Energy
•  Law of conservation of
energy- energy is neither
created or destroyed,
only changes state
–  Applies to closed systems
(E cannot leave or enter)
•  E.g. Swing
–  As you move back and
forth, energy is converted
from kinetic to potential
back to kinetic energy
continuously