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