energy transformations physics 11 – chapter 7. another try at humour…
TRANSCRIPT
Energy Transformations
Physics 11 – Chapter 7
Another try at humour…
Conservative and non-conservative forces:
Conservative forces:
oWork is independent of the path taken, it depends only on the final and initial positions
oWe can always/only associate a potential energy with conservative forces.
o This energy can be converted back into other forms of energy.
Examples: gravity, spring forces
Conservative and non-conservative forces:
Non-conservative forces:
oWork does depend on path.oA force is non-conservative if it causes a change in mechanical energy (mechanical energy is the sum of kinetic and potential energy).
oThis energy cannot be converted back into other forms of energy (irreversible).
oAn applied force can transfer energy into or out of the system.
Example: Frictional force
Sliding a book on a table
Review: Energy conversions/transformations: Energy can be changed from one form to another. Changes in the form of energy are called energy
conversions or transformations
Kinetic-Potential Energy Conversion
Roller coasters work because of the energy that is built into the system. Initially, the cars are pulled mechanically up the tallest hill, giving them a great deal of potential energy. From that point, the conversion between potential and kinetic energy powers the cars throughout the entire ride.
Kinetic-Potential Energy Conversions
As a basketball player throws the ball into the air, various energy conversions take place.
Ball slows down Ball speeds up
The Law of Conservation of Energy
Energy can be neither created nor destroyed by ordinary means. It can only be converted from one form
to another. If energy seems to disappear, then
scientists look for it – leading to many important discoveries.
Law of Conservation of Energy In 1905, Albert Einstein said that
mass and energy can be converted into each other.
He showed that if matter is destroyed, energy is created, and if energy is destroyed mass is created.
E = MC2
http://www.pbs.org/wgbh/nova/einstein/legacy.html
Law of conservation of mechanical energy:
Only with conservative forces.
Only with an isolated system (no energy added or removed):
The total mechanical energy of a system remains constant!
The final and initial energy of a system remain the same: Ei = Ef
Law of conservation of mechanical energy:
Prime (‘) used to represent conditions after process has completed
All units = Joules Don’t have to use all 3, depends on
situation
Ek + EP +Es = Ek’ + Ep
’ +Es’
Example #1:
What kinds of energy?
Kinetic and gravitational potential
(A) Ek + Ep = Ek’ + Ep’
½mv12
+ mgh1 = ½mv22
+ mgh2**because all terms have m, we can divide each by “m”and it will “disappear!!!”
½v12 + gh1 = ½v2
2 + gh2
what we know: v1=2.0m/s, h1=40.m, h2=25m, v2=?
½(2.0)2 + 9.81(40.0) = ½v2
2 + 9.81(25)
(2) + (392) = ½v22 + (245)
2+ 392 -245 = ½v22
149/0.5 = v22
√298 = v 17.3 m/s = v2
(B) Ek + Ep = Ek’ + Ep’ ½mv1
2 + mgh1 = ½mv2
2 + mgh2
**because all terms have m, we can divide each by “m” and it will “disappear!!!”
½v12 + gh1 = ½v2
2 + gh2
what we know: v1=2.0m/s, h1=40.m, v2=10.0 m/s, h2=?
½(2.0)2 + 9.81(40.0) = ½(10)2
+ 9.81h2
(2) + (392) = (50) + 9.81h2
2+ 392 -50 = 9.81h2
344/9.81 = h2
35.1m = h2
Try it :
Pg 287 #1-8