5.3

Types of Energy and the law of conservation of energy

p. 236-241

Energy Transformationssee p.236 chart

Any form of energy can be transferred from one place to another, or transformed from one form to another.

Whenever anything happens, energy is transferred or transformed.

Energy cannot be created or destroyed, and therefore, the total amount of energy in the system remains the same.

Section 5.3

Discussion: Forms and Types of Energy

Can you think of types of energy that each image represents?

Section 5.3

Forms and Types of Energy

electrical energy gravitational energy

elastic energysound energy

thermal energy

The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

9.7 Conservation of Energy

Efficiency of Energy Transformations

chemical → kinetic radiant → chemical

electrical → thermal chemical → kinetic

Conservation of Energy A system is a set of elements

that influence one another and on which external influences act.

Systems can be large (the universe) or small (an ant), and may contain many elements or as few as two.

When the energy interactions of a group of objects need to be analyzed, we often assume that these objects are isolated from all other objects in the universe.

Such a group is called an isolated system.

In an isolated system, the total amount of energy never changes.

Potential energy will become the kinetic energy of the arrow.

Conservation of Energy

As you draw back the arrow in a bow, you do work stretching the bow.

• The bow then has potential energy.

• When released, the arrow has kinetic energy equal to this potential energy.

• It delivers this energy to its target.

Conservation of Energy

Part of the PE of the wound spring changes into KE. The remaining PE goes into heating the machinery and the surroundings due to friction. No energy is lost.

Conservation of Energy

When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground.

Conservation of Energy

The water behind a dam has potential energy that is used to power a generating plant below the dam.

• The generating plant transforms the energy of falling water into electrical energy.

• Electrical energy travels through wires to homes where it is used for lighting, heating, cooking, and operating electric toothbrushes.

Conservation of Energy

Mechanical Energy Mechanical energy (Em) is the sum of the

kinetic energy, and the potential energy of a system:

Quantifying Energy TransformationsPhase 1: Before the dive p.238 Since motionless diver kinetic energy is zero Ek=1/2mv2

Gravitational potential energy is Eg=mgh (calculate…)

Total mechanical energy: Em=Ek+Ep (calculate…)

Phase 2: At half way point

Once diver leaves platform he will accelerate down at 9.8m/s2; half way the diver is 5m above water

Eg=mgh Ek=1/2mv2 Em=Ek+Ep

To find V: v2=vi2 +2a d (calculate)

Now sub into Ek=1/2mv2

Phase 3: At the water’s surface Eg=mgh Ek=1/2mv2 Em=Ek+Ep

When diver reaches after his height above water is 0m, so Eg=0 (calculate to show this)

Ek=1/2mv2 to find v: v2=vi2 +2a d (calculate)

Sample problem A toy-car track is set up on a tabletop as shown

below. The mass of the car is 25 g. Calculate the speed of the car at the bottom of

the ramp, assuming friction is negligible.

Homework

P. 241 practice #1 P. 241 questions #1-4

p. 241 question #3