1. Energy, Work & Power

2. The Nature of Energy energy different forms of energy
the ability to cause change
different forms of energy
electrical – making toast
chemical – stored in food
radiant – from the sun
thermal – warming of the atmosphere
energy plays a role in every activity you do

3. Kinetic Energy Kinetic Energy (KE) Kinetic Energy Equation KE = ½ mv2
energy a moving object has because of its motion
depends on the mass and speed of the object
Kinetic Energy Equation
KE = ½ mv2
KE = kinetic energy (J)
m = mass (kg)
v = speed (m/s)

4. EXAMPLE 1
KE= m= 15 kg v= 3 m/s

5. EXAMPLE 2
What is the kinetic energy of a 20 kg ball bouncing with a speed of 25 m/s?

6. Potential Energy energy does not always include motion
Potential Energy (PE)
stored energy an object has due to its position
Example
apple in a tree due to its height
when it falls, potential energy transferred to kinetic

7. Gravitational Potential Energy
Gravitational Potential Energy (GPE)
energy stored by objects due to their position above Earth’s surface
depends on distance above Earth’s surface and the object’s mass
Gravitational Potential Energy Equation
GPE = mgh
GPE = gravitational potential energy (J)
m = mass (kg)
g = 9.8 m/s2 (acceleration due to gravity)
h = height (m)

8. GPE = mgh
1. What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg and is 4 m above the ground?

9. GPE = mgh
2. An 80 kg diver jumps from a 10 m high platform. What is the gravitational potential energy of the diver halfway down?

10. GPE = mgh
3. A boulder on top of a mountain has mass of 66 kg and has a GPE of 1000 J. What is the height of the mountain/

11. Elastic Potential Energy
energy stored when an object is compressed or stretched
Example:
stretching a rubber band has stored elastic potential energy
when released, the potential energy changes to kinetic
Chemical Potential Energy
energy stored in chemical bonds
Example
gasoline
food

12. Conservation of Energy
when energy changes form, it is always conserved
a light transforms electrical energy to light and thermal energy
other devices that make use of electrical energy

13. Conversions between Kinetic and Potential
ME = PE + KE
ME = Mechanical Energy
PE = Potential Energy
KE= Kinetic Energy

14. Temperature and Heat temperature Kelvin (K)
measure of the average kinetic energy of all the particles in an object
Kelvin (K)
SI unit of temperature
a more commonly temperature scale used is Celsius (°C)
K=°C+273
°C=K – 273
Convert 20°C to K
20°C = 293 K

15. Thermal Energy thermal energy
sum of the kinetic and potential energy of the particles
is transferred by conduction, convection, and radiation
when the temperature of an object increases, the average kinetic energy of the particles in the object increases
as a result, the thermal energy increases

16. Water °C (Celsius) °F (Fahrenheit) K (Kelvin) boiling point – 100
freezing point – 0
°F (Fahrenheit)
boiling point – 212
freezing point – 32
K (Kelvin)
boiling point – 373
freezing point - 273

17. Heat temperature and heat are not the same, but they are related heat
thermal energy that flows from a warmer material to a cooler material
its unit is still in Joules (J)
heat always flows from warmer to cooler materials
heat depends on the amount of substance you have

18. Transferring Thermal Energy
thermal energy can be transferred three ways
conduction
convection
radiation

19. Conduction Conduction
transfer of thermal energy by collisions between particles in matter
conduction occurs because particles in matter are in constant motion
best conductors of heat  metals
there are electrons that are not bound to individual atoms, but can move easily through the metal

20. Convection Convection fluid – liquid or a gas Convection current
transfer of thermal energy in a fluid by the movement of warmer and cooler fluid from place to place
fluid – liquid or a gas
Convection current
transfer heat from warmer to cooler parts of the fluid

21. Radiation
Radiation
transfer of energy by electromagnetic waves
Energy transferred by radiation often is called radiant energy
These waves can travel through space even when no matter is present

22. Radiant Energy and Matter
materials that are light-colored reflect more radiant energy
dark-colored materials absorb more radiant energy
when radiant energy is absorbed by a material, the thermal energy of the material increases
insulator
a material in which heat flows slowly
wood, plastic, fiberglass, air

23. Work Work for work to occur, an object must move
transfer of energy that occurs when a force makes an object move
measured in joules
for work to occur, an object must move
the motion of the object must be in the same direction as the applied force on the object

24. Calculating Work
W = F d
W = Work (J)
F = Force (N)
d = distance (m)

25. A force of 75 N is exerted on a 45 kg couch and the couch is moved 5 m
A force of 75 N is exerted on a 45 kg couch and the couch is moved 5 m. How much work is done in moving the couch?

26. A lawn mower is pushed with a force of 80 N
A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing the lawn, what is the total distance the lawn mower was pushed?

27. The brakes on a car do 240,000 J of work in stopping the car
The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50 m while the brakes are being applied, what is the total force the brakes exert on the car?

28. Power
Power
amount of work done, or the amount of energy transferred, divided by the time required to do the work or transfer the energy
measured in watts (W)

29. Calculating Power
P = W t
W = Work (J)
P = Power (W)
t = time (s)

30. To lift a baby from a crib 50 J of work are done
To lift a baby from a crib 50 J of work are done. How much power is needed if the baby is lifted .5 s?

31. If a runner’s power is 130 W, how much work is done by the runner in 100 seconds?

32. The power produced by an electric motor is 500 W
The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?

33. You exert a vertical force of 72 N to lift a box to a height of 1
You exert a vertical force of 72 N to lift a box to a height of 1.0 m in a time of 2 seconds. How much power is used to lift the box?

34. Using Machines machine
device that makes doing work easier by increasing the force applied to an object, changing the direction of an applied force, or increasing the distance over which a force can be applied

35. Efficiency
the same amount of work is done with a machine, but the goal is to use less force to do the work
some machines change the direction of the force you apply
wedge-shaped blade of an ax
Using machines increases the efficiency of the work
Efficiency – the percentage of work input that becomes work output in a machine