physics chapter 11 energy chapter 11: energy 11.1 the many forms of energy 11.2 conservation of...
TRANSCRIPT
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)
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?