Download Mechanical energy transformations

Describing Energy
When something is able to change its surroundings or itself, it has
energy.
Energy is the ability to cause change.
Without energy nothing would ever change.
When work is done energy is transferred. So, energy can also be
described as the ability to do work.
Because of this, we measure energy in the same units as work,
which are...Joules.
When we calculate energy usage, or the amount of energy
transferred, we measure within a system.
A system is anything around which you can imagine a boundary.
a system can be a single object, like a tennis ball, or a group of
objects, like the solar system.
different forms of energy
There are many different forms of energy.
The four main types are:
Mechanical energy
Electrical energy
Chemical energy
Radiant energy
Regardless of the type of energy, it is all the same and is only
transferred between types.
Kinetic and Potential Energy
Objects in motion have energy.
kinetic energy is energy due to motion.
Even motionless objects have energy.
potential energy is energy that is stored due to the interactions
between objects.
there are 3 types of potential energy:
Elastic potential energy is energy that is stored by compressing or
stretching an object.
Chemical potential energy is energy due to chemical bonds.
Gravitational potential energy is energy due to gravitational forces
between objects.
An apple hanging from a tree has gravitational potential energy.
A stretched rubber band has elastic potential energy.
Food that will be digested has chemical potential energy.
A spring that is compressed has elastic potential energy.
A vase on a shelf has gravitational potential energy.
HW: p 119 # 20-23
Conservation of Energy
The Law of Conservation of Energy
As your height above the ground changes your gravitational
potential energy changes.
As your speed changes the kenetic energy changes
Think of a rollercoaster.
When the rollercoaster is high above the ground the GPE is
large and the KE is small.
When the cars are low, GPE is small and KE is large.
Energy changes back and forth between GPE and KE
The total energy remains constant
The law of conservation of energy states that energy cannot be
created or destroyed. Energy can only be converted from one
form to another or transferred from one place to another.
Conserving resources
You may have heard about or been told you to conserve energy.
These ideas are related to using energy resources (coal and oil)
wisely.
Energy transformations
Falling vase, a race car roaring by or the sun warming your skin
are all energy transformations.
Mechanical energy transformations
Bicycles, roller coasters, and swings can be described in terms
of mechanical energy.
Mechanical energy is the sum of the kinetic energy and the
potential energy of the objects in the system.
It includes kinetic energy, elastic potential energy and
gravitational potential energy.
It does not include nuclear energy, thermal energy or chemical
potential energy.
Mechanical energy is not the same as total energy because of the
types of energy that are not mechanical- these are still counted
in total energy.
Because of this mechanical energy is not necessarily conserved,
but it does usually remain constant or nearly constant. When this
is the case energy is only transformed between different types of
mechanical energy.
Falling objects
Think of an apple hanging from a treeThe apple earth system has gravitational potential energy.
It does not have kinetic energy while the apple is hanging from
the tree because the apple is not moving.
When the apple falls the gravitational potential energy
decreases, so it is transformed into kinetic energy as the apples
speed increases.
The mechanical energy does not change as the apple falls, the
potential energy that is lost is gained back as kinetic energy.
The form changes, but the amount of mechanical energy stays
the same
Projectile Motion
During projectile motion an object moves in a curved path (like
a base ball through the air- it arcs)
When a baseball is hit the ball is moving fast as the ball leaves
the bat, so the system’s kinetic energy is relatively large.
The ball’s speed decreases as it rises, so the KE decreases. But
as the ball goes higher the GPE increases.
At the top of the curve the GPE is ________ and the KE is
_________.
Then when the baseball falls the GPE decreases and the KE
increases
The mechanical energy of the system stays the same.
Swings
At the top of the swing you have high _______ and low
________
At the bottom of the swing curve you have high ________ and
low _________
Other energy transformations
The effect of frictionFriction and air resistance convert some of the mechanical
energy into thermal energy
Thermal energy is the energy of heat and hot objects
When this happens mechanical energy is not destroyed just
transformed into thermal energy, which is soon transferred into
the air
Transforming electrical energy
Many of the devices we use every day are transforming
electrical energy.
They transform it into light, heat (radiant energy), mechanical
energy and sound energy
Transforming chemical potential energy
Most of the transformation of CPE is in the form of fuel
Gasoline, or coal
Also, plants turn radiant energy from the sun into chemical
potential energy.
Power-how fast energy changes
Power is the rate at which energy is converted
P=e/t
Power is measured in watts (1 watt =1joule per second)
You transform 950 J of chemical energy into mechanical energy
to push a sofa. If you took 5 seconds to move the sofa what was
your power?
If a runner’s power is 400 W as she runs, how much chemical
energy does she convert into other forms in 10.0 minutes?
One horsepower is a unit of pwer equal to746 W. How much
energy can a 150 horsepower engine transform in 10.0 seconds?
P 127 # 28-33 by end of class