Transcript energy notes

Warm up problem
Mass: 5 kg.
Angle of ramp: 30 degrees
Length of ramp: 20 meters
Frictionless
ramp
Vf = ?
Energy
and
The Law of Conservation of Energy
Energy
• Energy is the ability to change or cause change.
• If there is no energy, there can be no change.
Most of our energy comes from
the sun
Light from the sun powers all plant growth
Plants in turn feed all living organisms
(either directly or indirectly)
• Plants also become our fossil fuels
•
•
Potential and Kinetic Energy
• Potential Energy
– Energy of what might
happen
– Has potential to
cause change
– Ex. Compressing a
spring, a ball lifted
above the floor
(gravity), etc.
Write these 4 scenarios down and predict
which shows Potential or Kinetic Energy?
1.
2.
3.
4.
A bow is drawn back.
An arrow is shot from the bow.
A boulder at rest at the top of a hill.
A boulder rolling at the bottom of a hill.
Which Have Potential or Kinetic Energy?
1.
2.
3.
4.
A bow is drawn back. PE
An arrow is shot from the bow. KE
A boulder at rest at the top of a hill. PE
A boulder rolling at the bottom of a hill. KE
How Much Energy?
•What is the potential energy of a 2-kilogram
potted plant that is on a 2.2 meter-high plant
stand?
•What is the kinetic energy of a 3-kilogram ball
that is rolling at 2 m/s?
Potential vs Kinetic
• Potential Energy (Ep)
– Increases when mass,
gravity or height
increases.
– If the height doubles,
Ep doubles
 Kinetic Energy (Ek)
Increases when mass
increases
— If the mass doubles, Ek
doubles
— If the velocity doubles,
Ek quadruples
(squared)
—
—
What if velocity were
tripled?
The Law of Conservation of
Energy
• The total energy is the same at all points
of an object’s path
Law of Conservation of Energy
• Energy can never be created or destroyed.
• Energy can be converted from one form into
another
– Kinetic to potential
– Potential to kinetic
– Kinetic to heat (from friction)
Total Energybefore = Total Energyafter
• A falling rock has a potential energy of 750 J
and kinetic energy of 450 J. A few seconds
later, it has a potential energy of 400 J.
Determine its new kinetic energy.
• A heavy weight that has a mass of 50 kg is
lifted using a rope and pulley to a height of 8
meters above a wood post. The rope is cut
and the weight falls to hit the post. What is
the speed of the weight on impact?
Hint: List the
Ep and Ek for
each position
of the weight.
• A 2,000 kg car moving with a speed of 5 m/sec
starts up a hill, and the accelerator and brakes
are not working! How high does the car roll
before it stops?
• A 0.057 kg car starts from rest at the top of a
0.133-meter hill. Find the speed when it
reaches the bottom of the hill.
• A 500 kg roller coaster car starts from rest at
the top of a 60-meter hill. Find the speed
when it is halfway to the bottom.