2. Energy

3. Energy (def.) the ability to do work. Unit is Joules.
Work and energy are interrelated.
Work must be done on an object to get it to move. Energy is needed to do the work!
Moving objects have energy and can do work.

4. Kinetic Energy
Kinetic energy (KE ) - the energy an object possesses due to its motion.

5. Kinetic Energy (units focus)
Kinetic energy is the energy of motion.
It can be calculated by using the following equation.
KE = ½ mv2
Joule = kg x (m/s)2
N = kg m/s2 x m
Nm = Joule
*If the object’s mass is doubled then the kinetic energy is doubled but if the velocity is doubled the kinetic energy is quadrupled!

6. Example 1

7. Lets talk about Quest and your Project
There is a quest up right now. It is due Jan 24, 11pm.
You have a project, titled, “You are Energy consumed”. It is on my webpage, it is your responsibility to print a copy this time. It is due Jan 29 in class.

8. Potential Energy
When work is done, a resistive force, potential energy (PE), is created equal in magnitude to the work done.
Gravitational PE = mgh
Potential energy is sometimes called stored kinetic energy.
Types
Gravitational potential energy
Elastic potential energy

9. Potential Energy Potential energy is stored energy.
Anytime a particle or object is forced to maintain a position in which it would not naturally exist has potential energy.
The natural position is known as equilibrium.
Types of Potential Energy (mechanical)
Gravitational Potential Energy
Elastic Potential Energy
Types of Potential Energy (non-mechanical)
Electrical Potential Energy
Chemical Potential Energy

10. Example 2

11. Elastic Potential Energy
The energy stored in an elastic material

12. Elasticity
Elasticity is the ability of a material to return to its original shape after being deformed.
F = kx
F = Force (N)
k = spring constant (N/m)
x = amount spring or material is stretched or compresses. (m)
a negative x means the spring is compressed
A positive x means the spring is stretched.

13. Elastic Potential Energy
The amount of elastic energy stored in a spring.
PEelastic = ½ kx2
Ee = elastic potential energy
k = spring constant
x = degree spring is stretched or compressed

14. Elastic PE
When a spring is compressed or stretched a distance x, work is done against the elastic force of the spring creating elastic PE.

15. Elastic PE spring constant
The resisting force of a compressed or stretched spring is directly proportional to the displacement. (Hooke’s Law)
spring constant

16. Mechanical Energy
(def) – The sum of the Kinetic and Potential energy of an object
You can have an energy system with both KE and PE present at the same time. (ignore friction)
Ex:Pendulum
A pendulum system shows the interaction of kinetic and potential energy

17. Pendulum at Rest: No kinetic and no potential energy

18. Pull the pendulum up: at its highest point, it has all potential energy and no kinetic energy

19. As it moves through its arc the pendulum’s PE is converted to KE
Both PE and KE are present at the same time.

20. At bottom of the pendulum’s arc all PE is converted to KE and the pendulum is moving at its highest velocity

21. KE provides energy for the pendulum to do work and push the pendulum upward against gravity

22. KE provides energy for the pendulum to do work and push the pendulum upward against gravity
At the top of the arc, all KE is converted to PE

23. Mechanical Energy
Without considering other energies (thermal and chemical energies are negligible),
ME = KE + PE
Energy is conserved

24. As an object falls, its PE is continuously converted to KE
KEf = PEi
As an upthrown object comes to a halt, KE is continuously converted to PE
PEf = KEi
By substitution then, where KE = GPE
½mv2 = mgh
½v2 = gh

25. Conservation Law
In the absence of friction or work, the total mechanical energy remains the same. This is statement is called the Law of Conservation of Mechanical Energy.

26. Transformation from GPE to KE of a Falling Object
100% GPE % KE
75 % GPE % KE
50 % GPE % KE
25% GPE % KE
0% GPE % KE

27. Example 3
What is the velocity of a dropped 4.5 kg bowling ball the instant it hits the ground if it is dropped from a height of 13.6 m?

28. Example 4
A diver dives off a platform 50 m above a tank of water. If he has a mass of 56 kg what will be his kinetic energy, potential energy and speed
10 m below the top of the platform
30 m below the top, and
the instant he hits the water?

29. Example 5

30. Example 6
An 8.0 kg flower pot falls from a window ledge 12.0 m above a sidewalk. (a) What is the kinetic energy of the pot just as it reaches the sidewalk? (b) Using energy considerations only, determine the speed of the pot just before it strikes the walk.

31. Example 7
(a) How much work is needed to hoist a 98 N sack of grain to a storage room 50 m above the ground floor of a grain elevator?
(b) What is the potential energy of the sack of grain at this height?
(c) The rope being used to lift the sack of grain breaks just as the sack reaches the storage room. What kinetic energy does the sack have just before it strikes the ground floor?