1. Chapter 5 Energy

2. Work Work is the transfer of energy through motion.
In order for work to occur, a force must be exerted through a distance.
Work depends on:
the amount of force exerted.
the distance over which the force is applied.

3. Work
When force is applied in the direction of motion, work can be calculated:
W = F x d
Work = Force x distance
Work is measured in Joules ( J )
1 Joule = 1 Newton • meter

4. Work continued
There are two factors that determine if work is being done:
something has to move
motion must be in the direction of the applied force
(even when you carry books across a level floor no work is done on the books b/c the force is holding the books up and the movement is horizontal)
Solve Practice Problems

5. Work
Work is only done when components of a force are parallel to displacement
Components of the force perpendicular do not do work.

6. Net work done by a net constant

8. Work There are three key ingredients to
Force
Displacement
cause
In order for a force to qualify as having done work on an object
there must be a displacement
the force must cause the displacement.

9. Work by a waiter

10. Calculating the Amount of Work Done by Forces Problems

11. 5-1 Energy & Work Kinetic and Potential Energy
Energy can’t be smelled or heard, but you can smell, see, and hear the effects of energy. Therefore, energy is the ability to cause change.
Energy has many forms: radiant, electrical, chemical, thermal, and nuclear.
The basic unit of energy is the joule (J), named for the British scientist James Prescott Joule.

12. Kinetic Energy Equation
Kinetic energy = ½ x mass x (speed) 2
KE=1/2 mv2

13. Kinetic energy is the energy of motion
Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or horizontal motion - has kinetic energy.

14. There are many forms of kinetic energy - vibrational (the energy due to vibrational motion), rotational (the energy due to rotational motion), and translational (the energy due to motion from one location to another).

15. Translational kinetic energy
The amount of translational kinetic energy that an object has depends upon two variables:
the mass (m) of the object
the speed (v) of the object

16. The kinetic energy is dependent upon the square of the speed.
The following equation is used to represent the kinetic energy (KE) of an object.
KE = 0.5 • m • v2
This equation reveals that the kinetic energy of an object is directly proportional to the square of its speed. That means that for a twofold increase in speed, the kinetic energy will increase by a factor of four, etc...

17. Kinetic energy is a scalar quantity
the kinetic energy of an object is completely described by magnitude alone.
the standard metric unit of measurement for kinetic energy is the Joule.

18. Problem
1. Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s.

19. 2. If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy?

20. 3. Missy Diwater, the former platform diver for the Ringling Brother's Circus, had a kinetic energy of J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed?

21. 4. A 900-kg compact car moving at 60 mi/hr has approximately Joules of kinetic energy. Estimate its new kinetic energy if it is moving at 30 mi/hr. (HINT: use the kinetic energy equation as a "guide to thinking.")

22. Kinetic & Potential Energy
Kinetic energy is energy in the form of motion. KE depends upon the mass and velocity of the moving object.
The greater the mass or the greater the velocity, the greater the KE, assuming the other components remains the same.
Potential energy is stored energy.
Gravitation PE depends on height.
Elastic PE depends on stretching/ compression.

23. Potential Energy
An object can store energy as the result of its position.
Potential energy is the stored energy of position possessed by an object.

24. Ex: Potential Energy
The heavy ball of a demolition machine is storing energy when it is held at an elevated position.

25. Ex: continued
A drawn bow is able to store energy as the result of its position. When assuming its usual position (i.e., when not drawn), there is no energy stored in the bow. Yet when its position is altered from its usual equilibrium position, the bow is able to store energy by virtue of its position.

26. 2 types of potential energy
gravitational potential energy
elastic potential energy.

27. Gravitational potential energy
is the energy stored in an object as the result of its vertical position or height.
The energy is stored as the result of the gravitational attraction of the Earth for the object.

28. Ex: Gravitational Potential Energy
The gravitational potential energy of the massive ball of a demolition machine is dependent on two variables - the mass of the ball and the height to which it is raised.

29. Mass and Gravitational PE
There is a direct relation between gravitational potential energy and the mass of an object.
More massive objects have greater gravitational potential energy.

30. Gravitational PE and Height
There is also a direct relation between gravitational potential energy and the height of an object.
The higher that an object is elevated, the greater the gravitational potential energy.

31. Gravitational PE equation:
PEgrav = mass • g • height
PEgrav = m *• g • h
In the above equation, m represents the mass of the object, h represents the height of the object and g represents the gravitational field strength (9.8 N/kg on Earth) - sometimes referred to as the acceleration of gravity.

37. What is Power

38. Power The standard metric unit of power is the Watt.
Rate at which work is done

39. Power formulas

40. Two physics students, Will N
Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most work? ______________ Which student delivers the most power? ______________ Explain your answers.

41. Answer
Ben and Will do the same amount of work. They apply the same force to lift the same barbell the same distance above their heads.
Yet, Ben is the most "power-full" since he does the same work in less time. Power and time are inversely proportional.

42. During a physics lab, Jack and Jill ran up a hill
During a physics lab, Jack and Jill ran up a hill. Jack is twice as massive as Jill; yet Jill ascends the same distance in half the time. Who did the most work? ______________ Who delivered the most power? ______________ Explain your answers.

43. Answer
Jack does more work than Jill. Jack must apply twice the force to lift his twice-as-massive body up the same flight of stairs. Yet, Jill is just as "power-full" as Jack. Jill does one-half the work yet does it one-half the time. The reduction in work done is compensated for by the reduction in time.

44. 3. A tired squirrel (mass of approximately 1 kg) does push- ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then determine its power.

45. Answer
The tired squirrel does 0.50 Joule of work in 2.0 seconds. The power rating of this squirrel is found by
P = W / t = (0.50 J) / (2.0 s) = 0.25 Watts

50. Power and Energy Consumption
It is easy to estimate the cost of energy for an electrical appliance if its power consumption rate and time used are known. The higher the power consumption rate and the longer the appliance is used, the greater the cost of that appliance.

51. The power consumption rate is P = W / t = E / t , where E is the energy supplied by the electricity company. So the energy consumed over a time t is
E = Pt

52. Electricity bills state the energy used in units of kilowatt- hours (kW ⋅ h), which is the product of power in kilowatts and time in hours. This unit is convenient because electrical power consumption at the kilowatt level for hours at a time is typical.

53. What is the cost of running a kW computer 6.00 h per day for 30.0 d if the cost of electricity is $0.120 per kW ⋅ h ?

54. Strategy
Cost is based on energy consumed; thus, we must find E from E = Pt and then calculate the cost. Because electrical energy is expressed in kW ⋅ h , at the start of a problem such as this it is convenient to convert the units into kW and hours.

55. Solution The energy consumed in kW ⋅ h is
E = Pt = (0.200 kW)(6.00 h/d)(30.0 d) (7.73)
= 36.0 kW ⋅ h,
cost = (36.0 kW ⋅ h)($0.120 per kW ⋅ h) = $4.32 per month.

56. The cost of using the computer in this example is neither exorbitant nor negligible. It is clear that the cost is a combination of power and time. When both are high, such as for an air conditioner in the summer, the cost is high.

57. Conservation of Energy
As many objects move, energy changes from kinetic to potential back to kinetic energy. Mechanical energy is the total amount of kinetic and potential energy in a system.
In any given situation, energy may change from one form to another, but the total amount of energy remains constant: energy is conserved.
The law of conservation of energy states that energy may change form but it cannot be created or destroyed under ordinary conditions.

58. Conservation of Energy continued
Even as objects move, they eventually slow down and stop. Where does the energy go?
Friction and air resistance are constantly acting on moving objects.
These forces cause some mechanical energy to be changed to thermal energy or heat.
Mechanical energy can be changed to thermal energy.

59. TME
As already mentioned, the mechanical energy of an object can be the result of its motion. The total amount of mechanical energy is merely the sum of the potential energy and the kinetic energy. This sum is simply referred to as the total mechanical energy (abbreviated TME).
TME = PE + KE

60. As discussed earlier, there are two forms of potential energy discussed in our course - gravitational potential energy and elastic potential energy. Given this fact, the above equation can be rewritten:
TME = PEgrav + PEspring + KE

61. 5-2 Temperature and Heat
Hot and cold are commons terms used to describe the temperature of materials. Are hot and cold definite? No, but, hot and cold are relative terms, they compare the temperature of two objects.
All objects are made of matter and these particles of matter are in constant motion. Since moving objects have KE, the faster they move the more KE they have.

62. Temperature and Heat
Temperature is a “measurement” of the average KE of the particles in a sample of matter.
As the particles in matter move faster, their average KE increases, therefore the temperature increases.
As the particles in matter move more slowly, their average KE decreases, and their temperature decreases.

63. Thermal Energy
Thermal energy is the total energy of the particles in a material, the total KE and PE in the material.
The KE is due to vibrations and movement of the particles of material.
The PE is determined by forces that act within or between the particles.
The more mass a material has at the same temperature, the greater its thermal energy.
Different kinds of matter have different thermal energies, due mainly to the ways the particles are arranged.

64. Heat
Energy flows from warmer objects to cooler objects. Heat is the “movement” of energy that flows from higher temperature objects to lower temp objects. (It never moves from cool to warm.)
Heat is measured in Joules since it is a form of energy and involves the transfer of energy.

65. 5-3 Energy from the Oceans & Earth
Ocean waters - the temperature change between the surface and deeper waters can be up to 15°C. This causes the waters to constantly move and churn. This movement can be used to generate electricity.

66. 5-3 Energy from the Oceans & Earth
Magma - is the molten rock that lies deep beneath Earth’s surface. Wells could be drilled to use this heat to generate electricity.
Both methods are very expensive, will take many years to produce practical results, and could also cause damage to the environment.
Read ch.

67. 5-4 Measuring Thermal Energy
Different materials need different amounts of heat to produce similar changes in their temperatures. The materials have different specific heats.
Specific heat (Cp) is the amount of energy it takes to raise the temperature of 1 kg of the material 1 degree Kelvin.

68. 5-4 Measuring Thermal Energy
Specific heat is measured in J/kg•K
(Joules per kilogram per degree Kelvin)
Specific heat depends on the chemical makeup of the substances.
Objects with high specific heats can absorb a lot of energy with little change in temperature.
Water and alcohol have high specific heats.
Objects with lower specific heats change temperature more quickly as they absorb heat.

69. Using Specific Heat
Specific heat can be used to measure changes in thermal energy using the formula:
Q = m x T x Cp T = Tfinal - Tinitial
change in thermal energy =
mass x change in temp x specific heat
When T is positive, temp increased, when T is negative, temp decreased.

70. Even though different substances have different specific heats, the mass and shapes of the substances also help determine the thermal energy characteristics - how the thermal energy moves.
Solve Practice Problems:
Section Review