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Physics Chapter 11 Energy

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**Chapter 11: Energy 11.1 The Many Forms of Energy**

11.2 Conservation of Energy

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**The Work-Energy Theorem**

Doing work on an object will increase or decrease its energy Work causes a change in energy that is equal to the work done W = E

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**The Work-Energy Theorem**

W = E E can be any form of energy In this chapter we will look at kinetic energy and potential energy

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**Kinetic Energy Kinetic Energy (KE) Energy of motion**

The energy of an object in motion.

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**Kinetic Energy Kinetic Energy (KE)**

What two things must an object have to have kinetic energy? An object must have mass and velocity to have kinetic energy

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**KE = ½ mv2 Kinetic Energy Kinetic Energy (KE) Equation:**

KE = kinetic energy (J) m = mass (kg) v = velocity (m/s)

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**Kinetic Energy Kinetic Energy (KE) Example:**

A 1.25 kg squirrel is running from a dog at 12.8 m/s. What is the squirrel’s kinetic energy? Answer: 102.4 J

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**Potential Energy Potential Energy Stored energy**

The energy an object has due to its position Several types of potential energy: Chemical energy Gravitational potential energy Elastic potential energy

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**Potential Energy Gravitational Potential Energy (GPE)**

The energy stored in an object has due to its position above a reference point (?) Reference point is usually the surface of the Earth

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**Potential Energy Gravitational Potential Energy (GPE)**

What three things does GPE depend upon? Mass, gravity and distance above reference point

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**GPE = mgh Potential Energy Gravitational Potential Energy (GPE)**

Equation: GPE = mgh m = mass (kg) g = 9.8 m/s2 h = height (m)

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**Potential Energy Gravitational Potential Energy (GPE) Example:**

A 95 kg woman is at the top of a mountain which is 1.5 km high. What is her gravitational potential energy? Answer: J

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**Potential Energy Elastic Potential Energy (EPE)**

The energy stored in an object that has been stretched or compressed Examples: Springs, rubber balls, slingshots, bows

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**11.2 Conservation of Energy**

When a system is closed (?) there is a relationship between all the types of energy within the system. The total amount of energy in a closed system is constant. (it is conserved) This is called the Law of Conservation of Energy

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**11.2 Conservation of Energy**

Law of Conservation of Mechanical Energy The mechanical energy (KE + PE) of a given system is constant if no other forms of energy are present. KE + PE is conserved

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**11.2 Conservation of Energy**

Law of Conservation of Mechanical Energy E = KE + PE or KEbefore + PEbefore = KEafter + PEafter

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**11.2 Conservation of Energy**

When a ball is held above the ground it has a certain amount of PE and no KE The total energy (E) of the system is equal to: E = KE + PE

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**11.2 Conservation of Energy**

When a ball is released and falls toward the ground it loses a certain amount of PE and gains a certain amount of KE, but E is still the same! The total energy (E) of the system is equal to: E = KE + PE And PE “lost” is equal to KE “gained”

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**11.2 Conservation of Energy**

Just before the ball reaches the ground it loses all the PE and gains KE, but E is still the same! The total energy (E) of the system is equal to: E = KE + PE And PE “lost” is equal to KE “gained”

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**11.2 Conservation of Energy**

What about when a ball is tossed upwards? (Remember: E = constant!) When is the kinetic energy the most? When is the potential energy the most? When is the mechanical energy the most?

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**11.2 Conservation of Energy**

A 0.75kg ostrich egg is held 22m above the Earth. Before it falls, what is its: Kinetic energy? 0 J Gravitational potential energy? 161.7 J Mechanical energy?

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**11.2 Conservation of Energy**

A 0.75kg ostrich egg is held 22m above the Earth. After it falls 11m (half way), what is its: Kinetic energy? 80.85 J Gravitational potential energy? Mechanical energy? 161.7 J

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**11.2 Conservation of Energy**

A 0.75kg ostrich egg is held 22m above the Earth. Just before it hits the ground, what is its: Kinetic energy? 161.7 J Gravitational potential energy? 0 J Mechanical energy?

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**11.2 Conservation of Energy**

A 0.75kg ostrich egg is held 22m above the Earth. Just before it hits the ground, what is its speed? Kinetic energy = J KE = 1/2mv2 161.7 = ½(0.75)v2 v = 20.8 m/s

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**11.2 Conservation of Energy**

So…. What is the relationship between KE, PE, and ME at all times during the egg’s fall?

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**11.2 Conservation of Energy**

If mechanical energy is conserved, where does it go when it is “lost” as a pendulum swings?

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**Collisions When two objects hit each other it is called a collision.**

There are two types of collisions: Elastic collision Inelastic collision

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**Collisions Elastic collision**

Collision between objects in which the kinetic energy of the system stays the same KEbefore = KEafter Usually between very hard objects and or very elastic objects What about momentum?

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**Collisions Inelastic collision**

Collision between objects in which the kinetic energy of the system changes KEbefore KEafter Usually between soft objects that deform. What about momentum?

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Collisions During an elastic collision both momentum and kinetic energy is conserved. During an inelastic collision momentum is conserved but kinetic energy is not. Where does the kinetic energy go?

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