Chapter 9.7

Chapter 9.7

Conservation of Energy

For moving objects such as cars:

The more kinetic energy it has, the more work is required to stop it.

Twice as much kinetic energy means twice as much work.

Brakes do work on wheels (you do work by pushing the brake pedal). When a car brakes, the work is the friction force (supplied by the brakes) multiplied by the distance over which the friction force acts.

KE is transformed by work (friction) into thermal energy, sound energy and larger-scale vibrations.

9.6 Work-Energy Theorem

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

For any system in its entirety—as simple as a swinging pendulum or as complex as an exploding galaxy—there is one quantity that does not change: energy. Energy may change form, but the total energy stays the same.

When energy is transformed, it is conserved, meaning that it will change form without losing its original amount of energy.

9.7 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.

9.7 Conservation of Energy

Elastic potential energy will become the kinetic energy of the arrow when the bow does work on the arrow.

9.7 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.

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.

9.7 Conservation of Energy

9.7 Conservation of Energy

Same energy transformation applies

The 2 J of heat can be called non-useful work (work that is not part of the object’s total mechanical energy).

10 J of PE does 8 J useful work on the arrow and 2 J of non-useful work on the molecules that compose the bow and string and arrow. The arrow has 8 J of KE.

Everywhere along the path of the pendulum bob, the sum of PE and KE is the same. Because of the work done against friction, this energy will eventually be transformed into heat.

9.7 Conservation of Energy

Non-useful work can also be called non-useful energy!

Useful vs. Non-Useful Energy

• Useful energy is total mechanical energy TME = PE + KE

• Non-useful energy are those forms that cause the TME to decrease. Otherwise known as non-mechanical energy!– Thermal energy (heat from friction)– Sound energy– Vibrations not related to the original motion of the object

• Example: friction decreases KE, causing the object to decrease in speed and causing the object and it surrounding to increase in temperature.

• Can also be referred to as useful and non-useful work.

9.7 Conservation of Energy

• Why does a tennis ball eventually stop bouncing?

• Eventually, all of the total mechanical energy is transformed into non-useful energy (heat, sound, movement of fibers)

50 J PE

50 J KE

New height less than before means less PE stored 35 J PE

Bounce!35 J KE

Bounce!

20 J PE

(bounce and so on!)

20 J KE

Slides showing transformation of KE and PE

• Source: http://www.physicsclassroom.com/mmedia/index.cfm

Watch how KE and gravitational PE transform

Where is the KE at the maximum?

Where is the PE at the maximum?

How is PE stored?

Watch the change in height vs. the change in speed!

How does the change in height affect KE and PE?

What happens to KE and TME when the brakes are applied? What work is being

done?

Watch the transfer of KE and PE.

What happens to the PE when the skier moves down the hill?

What happens to the KE and TME when the skier travels over the unpacked snow?

What work is done?

Same work, more force, less displacement (from left to right)

Each atom that makes up matter is a concentrated bundle of energy.

When the nuclei of atoms rearrange themselves, enormous amounts of energy can be released.

The sun shines because some of its nuclear energy is transformed into radiant energy.

In nuclear reactors, nuclear energy is transformed into heat.

9.7 Conservation of Energy

Enormous compression due to gravity in the deep, hot interior of the sun causes hydrogen nuclei to fuse and become helium nuclei.

• This high-temperature welding of atomic nuclei is called thermonuclear fusion.

• This process releases radiant energy, some of which reaches Earth.

• Part of this energy falls on plants, and some of the plants later become coal.

9.7 Conservation of Energy

• Another part supports life in the food chain that begins with microscopic marine animals and plants, and later gets stored in oil.

• Part of the sun’s energy is used to evaporate water from the ocean.

• Some water returns to Earth as rain that is trapped behind a dam.

9.7 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.

9.7 Conservation of Energy

What does the law of conservation of energy state?

9.7 Conservation of Energy