1. Chapter 5
Energy

2. Forms of Energy Mechanical Chemical Electromagnetic Nuclear
Focus for now
May be kinetic (associated with motion) or potential (associated with position)
Chemical
Electromagnetic
Nuclear

3. Some Energy Considerations
Energy can be transformed from one form to another
Essential to the study of physics, chemistry, biology, geology, astronomy
Can be used in place of Newton’s laws to solve certain problems more simply

4. Work Provides a link between force and energy
The work, W, done by a constant force on an object is defined as the product of the component of the force along the direction of displacement and the magnitude of the displacement

5. Work, cont. F is the magnitude of the force
Δ x is the magnitude of the object’s displacement
q is the angle between

6. Work
Visual

7. Work, cont. This gives no information about Work is a scalar quantity
the time it took for the displacement to occur
the velocity or acceleration of the object
Work is a scalar quantity

8. Units of Work SI US Customary Newton • meter = Joule foot • pound
N • m = J
J = kg • m2 / s2
US Customary
foot • pound
ft • lb
no special name

9. More About Work
The work done by a force is zero when the force is perpendicular to the displacement
cos 90° = 0
If there are multiple forces acting on an object, the total work done is the algebraic sum of the amount of work done by each force

10. More About Work, cont. Work can be positive or negative
Positive if the force and the displacement are in the same direction
Negative if the force and the displacement are in the opposite direction

11. When Work is Zero Displacement is horizontal Force is vertical
cos 90° = 0

12. Work Can Be Positive or Negative
Work is positive when lifting the box
Work would be negative if lowering the box
The force would still be upward, but the displacement would be downward

13. Work and Dissipative Forces
Work can be done by friction
The energy lost to friction by an object goes into heating both the object and its environment
Some energy may be converted into sound
For now, the phrase “Work done by friction” will denote the effect of the friction processes on mechanical energy alone

14. Example 1
A weight lifter lifts a 350-N set of weights from ground level to a position over his head, a vertical distance of 2.00 m. How much work does the weight lifter do, assuming he moves the weights at constant speed?

15. Example 2
A shopper in a supermarket pushes a cart with a force of 35 N directed at an angle of 25° downward from the horizontal. Find the work done by the shopper as she moves down a 50-m length of aisle.

16. Example 3
Starting from rest, a 7.00-kg block slides 2.90 m down a rough 35.0° incline. The coefficient of kinetic friction between the block and the incline is μk = Determine (a) the work done by the force of gravity, (b) the work done by the friction force between block and incline, and (c) the work done by the normal force.

17. Kinetic Energy Energy associated with the motion of an object
Scalar quantity with the same units as work
Work is related to kinetic energy

18. Kinetic Energy
Visual

19. Work-Kinetic Energy Theorem
When work is done by a net force on an object and the only change in the object is its speed, the work done is equal to the change in the object’s kinetic energy
Speed will increase if work is positive
Speed will decrease if work is negative

20. Work and Kinetic Energy
An object’s kinetic energy can also be thought of as the amount of work the moving object could do in coming to rest
The moving hammer has kinetic energy and can do work on the nail

21. Work-Kinetic Energy
Visual

22. Example 4
A 70-kg base runner begins his slide into second base when he is moving at a speed of 4.0 m/s. The coefficient of friction between his clothes and Earth is He slides so that his speed is zero just as he reaches the base. (a) How much mechanical energy is lost due to friction acting on the runner? (b) How far does he slide?

23. Example 5
An outfielder throws a kg baseball at a speed of 40.0 m/s and an initial angle of 30.0°. What is the kinetic energy of the ball at the highest point of its motion?

24. Example 6
On a frozen pond, a 10-kg sled is given a kick that imparts to it an initial speed of v0 = 2.0 m/s. The coefficient of kinetic friction between sled and ice is μk = Use the work–energy theorem to find the distance the sled moves before coming to rest.

25. Types of Forces There are two general kinds of forces Conservative
Work and energy associated with the force can be recovered
Nonconservative
The forces are generally dissipative and work done against it cannot easily be recovered

26. Conservative Forces
A force is conservative if the work it does on an object moving between two points is independent of the path the objects take between the points
The work depends only upon the initial and final positions of the object
Any conservative force can have a potential energy function associated with it

27. More About Conservative Forces
Examples of conservative forces include:
Gravity
Spring force
Electromagnetic forces
Potential energy is another way of looking at the work done by conservative forces

28. Nonconservative Forces
A force is nonconservative if the work it does on an object depends on the path taken by the object between its final and starting points.
Examples of nonconservative forces
kinetic friction, air drag, propulsive forces

29. Friction as a Nonconservative Force
The friction force is transformed from the kinetic energy of the object into a type of energy associated with temperature
The objects are warmer than they were before the movement
Internal Energy is the term used for the energy associated with an object’s temperature

30. Friction Depends on the Path
The blue path is shorter than the red path
The work required is less on the blue path than on the red path
Friction depends on the path and so is a non-conservative force

31. Potential Energy
Potential energy is associated with the position of the object within some system
Potential energy is a property of the system, not the object
A system is a collection of objects interacting via forces or processes that are internal to the system

32. Work and Potential Energy
For every conservative force a potential energy function can be found
Evaluating the difference of the function at any two points in an object’s path gives the negative of the work done by the force between those two points

33. Gravitational Potential Energy
Gravitational Potential Energy is the energy associated with the relative position of an object in space near the Earth’s surface
Objects interact with the earth through the gravitational force
Actually the potential energy is for the earth-object system

34. Potential Energy
Visual

35. Work and Gravitational Potential Energy
PE = mgy
Units of Potential Energy are the same as those of Work and Kinetic Energy

36. Work-Energy Theorem, Extended
The work-energy theorem can be extended to include potential energy:
If other conservative forces are present, potential energy functions can be developed for them and their change in that potential energy added to the right side of the equation

37. Reference Levels for Gravitational Potential Energy
A location where the gravitational potential energy is zero must be chosen for each problem
The choice is arbitrary since the change in the potential energy is the important quantity
Choose a convenient location for the zero reference height
often the Earth’s surface
may be some other point suggested by the problem
Once the position is chosen, it must remain fixed for the entire problem

38. Example 7
Find the height from which you would have to drop a ball so that it would have a speed of 9.0 m/s just before it hits the ground.

39. Example 8
A daredevil on a motorcycle leaves the end of a ramp with a speed of 35.0 m/s as in Figure P5.23. If his speed is 33.0 m/s when he reaches the peak of the path, what is the maximum height that he reaches? Ignore friction and air resistance.

40. Conservation of Mechanical Energy
Conservation in general
To say a physical quantity is conserved is to say that the numerical value of the quantity remains constant throughout any physical process
In Conservation of Energy, the total mechanical energy remains constant
In any isolated system of objects interacting only through conservative forces, the total mechanical energy of the system remains constant.

41. Conservation of Energy, cont.
Total mechanical energy is the sum of the kinetic and potential energies in the system
Other types of potential energy functions can be added to modify this equation

42. Conservation of Energy
Visual

43. Problem Solving with Conservation of Energy
Define the system
Select the location of zero gravitational potential energy
Do not change this location while solving the problem
Identify two points the object of interest moves between
One point should be where information is given
The other point should be where you want to find out something

44. Problem Solving, cont Verify that only conservative forces are present
Apply the conservation of energy equation to the system
Immediately substitute zero values, then do the algebra before substituting the other values
Solve for the unknown(s)

45. Work-Energy With Nonconservative Forces
If nonconservative forces are present, then the full Work-Energy Theorem must be used instead of the equation for Conservation of Energy
Often techniques from previous chapters will need to be employed

46. Potential Energy Stored in a Spring
Involves the spring constant, k
Hooke’s Law gives the force
F = - k x
F is the restoring force
F is in the opposite direction of x
k depends on how the spring was formed, the material it is made from, thickness of the wire, etc.

47. Spring Constant
Visual

48. Potential Energy in a Spring
Elastic Potential Energy
related to the work required to compress a spring from its equilibrium position to some final, arbitrary, position x

49. Work-Energy Theorem Including a Spring
Wnc = (KEf – KEi) + (PEgf – PEgi) + (PEsf – PEsi)
PEg is the gravitational potential energy
PEs is the elastic potential energy associated with a spring
PE will now be used to denote the total potential energy of the system

50. Conservation of Energy Including a Spring
The PE of the spring is added to both sides of the conservation of energy equation
The same problem-solving strategies apply

51. Nonconservative Forces with Energy Considerations
When nonconservative forces are present, the total mechanical energy of the system is not constant
The work done by all nonconservative forces acting on parts of a system equals the change in the mechanical energy of the system

52. Nonconservative Forces and Energy
In equation form:
The energy can either cross a boundary or the energy is transformed into a form of non-mechanical energy such as thermal energy

53. Transferring Energy By Work By applying a force
Produces a displacement of the system

54. Transferring Energy Heat
The process of transferring heat by collisions between molecules
For example, the spoon becomes hot because some of the KE of the molecules in the coffee is transferred to the molecules of the spoon as internal energy

55. Transferring Energy Mechanical Waves
A disturbance propagates through a medium
Examples include sound, water, seismic

56. Transferring Energy Electrical transmission
Transfer by means of electrical current
This is how energy enters any electrical device

57. Transferring Energy Electromagnetic radiation
Any form of electromagnetic waves
Light, microwaves, radio waves

58. Notes About Conservation of Energy
We can neither create nor destroy energy
Another way of saying energy is conserved
If the total energy of the system does not remain constant, the energy must have crossed the boundary by some mechanism
Applies to areas other than physics

59. Example 9
Tarzan swings on a 30.0-m-long vine initially inclined at an angle of 37.0° with the vertical. What is his speed at the bottom of the swing (a) if he starts from rest? (b) if he pushes off with a speed of 4.00 m/s?

60. Example 10
A projectile is launched with a speed of 40 m/s at an angle of 60° above the horizontal. Use conservation of energy to find the maximum height reached by the projectile during its flight.

61. Example 11
A kg block is placed on a light vertical spring (k = 5.00 × 103 N/m) and pushed downwards, compressing the spring m. After the block is released, it leaves the spring and continues to travel upwards. What height above the point of release will the block reach if air resistance is negligible?

62. Example 12
Starting from rest, a 10.0-kg block slides 3.00 m down to the bottom of a frictionless ramp inclined 30.0° from the floor. The block then slides an additional 5.00 m along the floor before coming to a stop. Determine (a) the speed of the block at the bottom of the ramp, (b) the coefficient of kinetic friction between block and floor, and (c) the mechanical energy lost due to friction.

63. Power
Often also interested in the rate at which the energy transfer takes place
Power is defined as this rate of energy transfer
SI units are Watts (W)

64. Power, cont. US Customary units are generally hp
Need a conversion factor
Can define units of work or energy in terms of units of power:
kilowatt hours (kWh) are often used in electric bills
This is a unit of energy, not power

65. Power
Visual

66. Example 13
While running, a person dissipates about 0.60 J of mechanical energy per step per kilogram of body mass. If a 60-kg person develops a power of 70 W during a race, how fast is the person running? (Assume a running step is 1.5 m long.)

67. Example 14
A 1.50 × 103-kg car starts from rest and accelerates uniformly to 18.0 m/s in 12.0 s. Assume that air resistance remains constant at 400 N during this time. Find (a) the average power developed by the engine and (b) the instantaneous power output of the engine at t = 12.0 s, just before the car stops accelerating.

68. Center of Mass
The point in the body at which all the mass may be considered to be concentrated
When using mechanical energy, the change in potential energy is related to the change in height of the center of mass

69. Work Done by Varying Forces
The work done by a variable force acting on an object that undergoes a displacement is equal to the area under the graph of F versus x

70. Spring Example
Spring is slowly stretched from 0 to xmax
W = ½kx²

71. Spring Example, cont.
The work is also equal to the area under the curve
In this case, the “curve” is a triangle
A = ½ B h gives W = ½ k x2

72. Example 15
An object is subject to a force Fx that varies with position as in Figure P5.56. Find the work done by the force on the object as it moves (a) from x = 0 to x = 5.00 m, (b) from x = 5.00 m to x = 10.0 m, and (c) from x = 10.0 m to x = 15.0 m. (d) What is the total work done by the force over the distance x = 0 to x = 15.0 m?