Physics Chapter 11 Energy Chapter 11: Energy 11.1 The Many Forms of Energy 11.2 Conservation of Energy.

Physics

Chapter 11

Energy

Chapter 11: Energy 11.1 The Many Forms of Energy 11.2 Conservation of Energy

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

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

Kinetic EnergyKinetic Energy (KE) Energy of motion The energy of an

object in motion.

Kinetic EnergyKinetic Energy (KE)

What two things must an object have to have kinetic energy?

An object must have mass and velocity to have kinetic energy

Kinetic EnergyKinetic Energy (KE)

Equation:

KE = ½ mv2

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

Kinetic EnergyKinetic 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

Potential EnergyPotential 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

Potential EnergyGravitational 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

Potential EnergyGravitational Potential

Energy (GPE)

What three things does GPE depend upon?

Mass, gravity and distance above reference point

Potential EnergyGravitational Potential

Energy (GPE)

Equation:

GPE = mgh m = mass (kg) g = 9.8 m/s2

h = height (m)

Potential EnergyGravitational 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: 1396500 J

Potential EnergyElastic Potential

Energy (EPE) The energy stored in

an object that has been stretched or compressed

Examples: Springs, rubber balls,

slingshots, bows

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

11.2 Conservation of EnergyLaw 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

11.2 Conservation of EnergyLaw of Conservation of

Mechanical Energy

E = KE + PEor

KEbefore + PEbefore =

KEafter + PEafter

11.2 Conservation of EnergyWhen 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

11.2 Conservation of EnergyWhen 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 + PEAnd PE “lost” is equal to KE

“gained”

11.2 Conservation of EnergyJust 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”

11.2 Conservation of EnergyWhat 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?

11.2 Conservation of EnergyA 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? 161.7 J

11.2 Conservation of EnergyA 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? 80.85 J

Mechanical energy? 161.7 J

11.2 Conservation of EnergyA 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? 161.7 J

11.2 Conservation of EnergyA 0.75kg ostrich egg is held

22m above the Earth. Just before it hits the ground,

what is its speed? Kinetic energy = 161.7 J KE = 1/2mv2 161.7 = ½(0.75)v2

v = 20.8 m/s

11.2 Conservation of EnergySo….

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

11.2 Conservation of EnergyIf mechanical energy is

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

Collisions When two objects hit

each other it is called a collision.

There are two types of collisions: Elastic collision Inelastic collision

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?

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?

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?