chapter 7 - conservation of energy and momentum

8/13/2019 Chapter 7 - Conservation of Energy and Momentum

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UNIT 3Momentum & Energy


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EFFICIENCY

Efficiency is the ratio of useful energy or work

output to the total energy or work output.


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EFFICIENCY

A model rocket engine contains explosives storing

3.5*10^3J of chemical potential energy. The stores

energy is transformed into gravitational potential

energy at the top of a rockets flight path. Calculate

how efficiently the rocket transforms storedchemical energy into gravitational potential energy

if the 0.500kg rocket is propelled to a height of

1.00*10^2m.


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EFFICIENCY


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CHAPTER 17CONSERVATION OF

ENERGY AND MOMENTUM

Law of conservation of energy: Energy can not becreated or destroyed, or may only be transformed.

Thus, the total energy of an isolated system must

be constant over time.


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ENERGY TRANSFORMATIONS

Youve talked about kinetic, potential, and

gravitational energyall mechanical energies. Now

we will talk about what happens to the energy after

youve done work on the object.

If you shoot a hockey puck down an ice surface,

what will eventually happen?

It has lost the mechanical energy that weve given

it!


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CONSERVATIVE AND NON-

CONSERVATIVE FORCES

If you lift a book one meter above a table and

release it, what happens?

If you push a book across a table, will itautomatically return to its original spot?

What are we doing work against in the first case?

And the second?


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CONSERVATIVE AND NON-

CONSERVATIVE FORCES

Now, if you lift the book one meter above the table,

and then carry it across the room, have you done

more work on the book than in the first case?

If you push the book from one side to the other, and

then push it back to its original position, have you

done more work than before?


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CONSERVATION OF MECHANICAL

ENERGY

When all of the work done throughout a process is

done by conservative forces, the total mechanical

energy of the system before the process is equal to

the total mechanical energy at the end of the

process.

Ek + Eg + Ee= Ek + Eg + Ee

k = kinetic

g = gravitational

e = elastic


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ENERGY

What kind of energy transformation is happening if

you drop a rock?

What happens the total energy?

What is happening to the different energies

throughout the process?


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ENERGY

A skier is gliding along with a speed of 2.00m/s at

the top of a ski hill, 40.0m high. The skier then

begins to slide down the icy (frictionless) hill.

a. What will be the skiers speed at a height of 25.0m?

b. At what height will the skier have a speed of 10.0m/s?


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ENERGY


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ENERGY


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ENERGY

Homework problems: pg. 287, #1-4


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ELASTIC POTENTIAL ENERGY AND

KINETIC ENERGY

Youve leaned what elastic potential energy is and

how to express it when stored in a stretched or

compressed spring:

Now, what are some practical examples of

transforming elastic potential energy into kinetic

energy?


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KINETIC ENERGY


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KINETIC ENERGY

A low friction cart with a mass of 0.25kg travels

along a horizontal track and collides head on with a

spring that has a spring constant of

155N/m. If the spring was compressed by 6.0cm,

how fast was the cart initially travelling?


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CONSERVATION OF TOTAL ENERGY


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A system is any object or group of objects.

An internal force is any force exerted on any objectin the system due to another object in the system.

An external force is any force exerted by an object

that is not a part of the system on an object withinthe system.


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An open system can exchange both matter and

energy with its surroundings.

A closed system can exchange energy, but notmatter, with its surroundings.

An isolated system can not exchange energy or

matter with its surroundings - nothing can enter orleave.


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You often hear that a system lost energy. Since

the energy can not be created or destroyed, what

happens to the energy?


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We can now state the law of conservation of energy

mathematically:

The work done by non-conservative forces is the

difference between the final mechanical energy andthe initial mechanical energy of a system.


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A roller-coaster with a mass of 200.0kg is moving to the right at a

speed of 4.00m/s at point A, 15.00m above the ground. The

car then heads down the slope towards point B, 6.00m above

the ground. If 3.40*10^3J of work are done by friction between

points A and B, determine the speed of the car at point B.


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Example: A 65.0kg skydiver steps out from a hot air

balloon that is 5.00*10^2m above the ground. After

free-falling a short distance, she deploys her

parachute, finally reaching the ground with a

velocity of 8.00m/s.

a. What is the gravitational potential energy

of the skydiver, relative to the ground,

before she jumps?b. What is the kinetic energy just before she

lands on the ground?

c. How much work did the non-conservative

frictional force do?


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CONSERVATION OF MOMENTUM

Elastic collisionskinetic energy is conserved.

Inelastic collisionskinetic energy is not

conserved.

Newtons third law of motion states that for every

action force on object B due to object A, there is a

reaction force equal in magnitude (but opposite

direction) acting on object A due to object B.


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So how do we derive the conservation of

momentum law from Newtons third law? We use

the impulse-momentum theorem!


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The conservation of momentum law states that the

total momentum of two objects before a collision is

the same as the total momentum of the same two

objects after the collision.


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We all know that systems are rarely perfectly

isolated, and that immediately after a collision,

frictional forces and interactions with other objects

change the momentum of the objects.

So is this law still useful?


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Yes! We can still use the conservation of

momentum law to describe a system from the

instant before to the instant after a collision.


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A 1.75*10^4kg boxcar is rolling down a track toward

a stationary boxcar that has a mass of 2.00*10^4kg.

Just before the collision, the first boxcar is moving

east at 5.45m/s. When the boxcars collide, they

lock together and continue down the track. What isthe velocity of the two boxcars after the collision?


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A 0.250kg billiard ball moving at 5.00m/s collides

head-on with a stationary, 0.800kg steel ball. If the

billiard ball bounces directly backwards at 2.62m/s,

was the collision elastic?


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REAL LIFE EXAMPLES

What are some examples that make this important

for everyday events?

Homework: pg. 327 # 2,7,8

pg. 332 # 38, 42, 44, 46, 49

chapter 7 - conservation of energy and momentum

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