1. Physics
Chapter 11
Energy

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

3. 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

4. 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

5. Kinetic Energy Kinetic Energy (KE) Energy of motion
The energy of an object in motion.

6. 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

7. KE = ½ mv2 Kinetic Energy Kinetic Energy (KE) Equation:
KE = kinetic energy (J)
m = mass (kg)
v = velocity (m/s)

8. 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

9. 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

10. 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

11. Potential Energy Gravitational Potential Energy (GPE)
What three things does GPE depend upon?
Mass, gravity and distance above reference point

12. GPE = mgh Potential Energy Gravitational Potential Energy (GPE)
Equation:
GPE = mgh
m = mass (kg)
g = 9.8 m/s2
h = height (m)

13. 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

14. Potential Energy Elastic Potential Energy (EPE)
The energy stored in an object that has been stretched or compressed
Examples:
Springs, rubber balls, slingshots, bows

15. 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

16. 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

17. 11.2 Conservation of Energy
Law of Conservation of Mechanical Energy
E = KE + PE or
KEbefore + PEbefore =
KEafter + PEafter

18. 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

19. 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”

20. 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”

21. 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?

22. 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?

23. 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

24. 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?

25. 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

26. 11.2 Conservation of Energy
So….
What is the relationship between KE, PE, and ME at all times during the egg’s fall?

27. 11.2 Conservation of Energy
If mechanical energy is conserved, where does it go when it is “lost” as a pendulum swings?

28. Collisions When two objects hit each other it is called a collision.
There are two types of collisions:
Elastic collision
Inelastic collision

29. 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?

30. 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?

31. 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?