1 / 57

Chapter 5 Work and Energy p. 159 in Holt Physics Text

Chapter 5 Work and Energy p. 159 in Holt Physics Text. General Physics – Mrs. Dimler. What to expect.

duante
Download Presentation

Chapter 5 Work and Energy p. 159 in Holt Physics Text

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 5Work and Energyp. 159 in Holt Physics Text General Physics – Mrs. Dimler

  2. What to expect In this chapter, you will learn about WORK and different types of ENERGY that are relevant to mechanics. KINETIC ENERGY, which is associated with motion, and POTENTIAL ENERGY, which is related to an object’s position, are two forms of energy that you will study.

  3. KEY TERMS from Chapter 5 Work Kinetic Energy Work-Kinetic Energy Theorem Potential Energy Gravitational Potential Energy Elastic Potential Energy Spring Constant Mechanical Energy Power

  4. WORK – Everyday sense Work in everyday sense means to do something that takes physical or mental effort.

  5. WORK – In Physics • WORK– when a force acting on an object causes a displacement of that object • Work = Force x Displacement (W=Fd) • Work is measured in Joules (J) – p. 161 of textbook • 1 Joule = 1 Newton x 1 Meter • Work is done only when the components of a force acting on an object are parallel to displacement of the object

  6. Does the following represent an example of WORK? Applying a force to a brick wall and becoming exhausted

  7. Does the following represent an example of WORK? A rocket accelerating

  8. Net Work • Net Work Done by a Constant Net Force: Wnet = Fnet· d · cosθ Where θ is the angle between the force and the displacement vector

  9. The sign of WORK is IMPORTANT! Work is a SCALAR quantity, but it can be positive or negative. Positive Work – Component of force is in the same direction as displacement Negative Work – Component of force is in the direction opposite the displacement

  10. W = Fdcosθ F d θ = 0

  11. W = Fdcosθ F d θ = 180º

  12. W = Fdcosθ F d θ = 90º

  13. Is WORK Positive or Negative? A. The road exerts a friction force on a speeding car skidding to a stop NEGATIVE B. A rope exerts a force on a bucket as the bucket is raised up a well. POSITIVE C. Air exerts a force on a parachute as the parachutist falls to earth NEGATIVE

  14. Critical Thinking Determine whether work is being done in each of the following examples: • A train engine pulling a loaded boxcar initially at rest yes • A tug of war that is evenly matched no • A crane lifting a car yes

  15. BONUS Question If a neighbor pushes a lawn mower four times as far as you do but exerts only half the force, which one of you does more work and by how much?

  16. Examples: Work Problems A tugboat pulls a ship with a constant net horizontal force of 5,000 Newtons and causes the ship to move through a harbor. How much work is done on the ship if it moves a distance of 3.00 km? A weight lifter lifts a set of weights a vertical distance of 2.00 m. If a constant net force of 350 N is exerted on the weights, what is the net work done on the weights? 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 on the cart as the shopper moves along a 50 m length of aisle. If 2.0 J of work is done in raising a 180 g apple, how far is it lifted?

  17. Review WORK • Force is in direction of motion: POSITIVE WORK • Force opposes motion: NEGATIVE WORK • Force is 90˚ to motion: NO WORK • Object is not in motion: NO WORK

  18. Kinetic Energy • Kinetic Energy The energy of an object that is due to the object’s motion is called kinetic energy. • Kinetic Energy (KE) is a scalar quantity and it depends on an object’s speed and its mass. Kinetic Energy = x mass x (speed)² • Unit of Kinetic Energy is the Joule (J)

  19. KE Practice Problems A 6.0 kg cat runs after a mouse at 10.0 m/s. What is the cat’s kinetic energy? A 7.00 kg bowling ball moves at 3.00 m/s. How fast must a 2.45 g table-tennis ball move in order to have the same kinetic energy as the bowling ball?

  20. Work-Kinetic Energy Theorem • Work-Kinetic Energy Theorem The net work done by all the forces acting on an object is equal to the change in the object’s kinetic energy. • The net work done on a body equals its change in kinetic energy. Wnet= ∆KE = ½ mvf2– ½ mvi2 Net Work = “Change in” Kinetic Energy Note: When using this theorem, you must include all the forces that do WORK on the object in calculating the net work done.

  21. KE Sample Problem (p. 167) Work-Kinetic Energy Theorem On a frozen pond, a person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is 0.10?

  22. KE Sample Problem continued Work-Kinetic Energy Theorem Given: m = 10.0 kg vi = 2.2 m/s vf = 0 m/s µk = 0.10 Unknown: d = ?

  23. KE Sample Problem continued Note: This problem can be solved using the definition of WORKand the Work-Kinetic Energy Theorem together. Wnet= Fnetdcosq The net work done on the sled is provided by the force of kinetic friction. Wnet= Fkdcosq = µkmgdcosq

  24. KE Sample Problem continued The force of kinetic friction is in the direction opposite d, so θ= 180°. Because the sled comes to rest, the final kinetic energy is zero. Wnet = ∆KE = KEf - KEi = –(1/2)mvi2 Use the work-kinetic energy theorem, and solve for d. –(1/2)mvi2 = µkmgd cosθ

  25. KE Sample Problem answer

  26. Potential Energy Potential Energy is the energy associated with an object because of the position, shape, or condition of the object. Unit of measure for Potential Energy is the Joule (J) Different types of PE: -Gravitational PE -Elastic PE -Chemical PE -Electrical PE

  27. Potential Energy continued • Gravitational Potential Energyis the potential energy stored in the gravitational fields of interacting bodies. • Gravitational Potential Energydepends on height from a zero level. PEg= mgh gravitational PE = mass  free-fall acceleration  height

  28. Potential Energy Problem What is the gravitational potential energy of a 141 lb. person at 3.31 miles above sea level? (1lb. = 4.448N; 1mile = 1,609.34meters)

  29. Potential Energy continued Elastic potential energy is the energy available for use when a deformed elastic object returns to its original configuration. The symbol k is called the spring constant

  30. Spring Constant • k = spring constant is a measure of the spring’s resistance to being compressed or stretched. • Units for k is (N/m) • Flexible springs have small k, whereas stiff springs have a large k.

  31. Elastic Potential Energy

  32. PE Sample Problem When a 2.00kg mass is attached to a vertical spring, the spring is stretched 10.0cm such that the mass is 50 cm above the table. A. What is the gravitational PE associated with this mass relative to the table? B. What is the spring’s elastic PE if k=400.0 N/m? C. What is the total PE of this system?

  33. PE Sample Problem II (p. 171) Potential Energy A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight of the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?

  34. PE Sample Problem continued Potential Energy Problem Given: m= 70.0 kg k = 71.8 N/m g = 9.81 m/s2 h = 50.0 m – 44.0 m = 6.0 m x = 44.0 m – 15.0 m = 29.0 m PE = 0 J at river level Unknown:PEtot= ?

  35. PE Sample Problem continued Choose an equation or situation: The zero level for gravitational potential energy is chosen to be at the surface of the water. The total potential energy is the sum of the gravitational and elastic potential energy.

  36. PE Sample Problem continued Calculate - Substitute the values into the equations and solve:

  37. Conservation of Energy What does Conservation mean? When we say that something is conserved, we mean that it remains constant.

  38. Classifying Energy Mechanical Energy • Kinetic Energy • Potential Energy • Gravitational • Elastic Non-mechanical Energy • Nuclear Energy • Chemical Energy • Internal Energy • Electrical Energy

  39. Mechanical Energy • Mechanical Energy is the sum of kinetic energy and all forms of potential energy associated with an object or group of objects. ME = KE + ∑PE • Mechanical energy is often conserved. MEi= MEf initial mechanical energy = final mechanical energy (in the absence of friction)

  40. Pendulum

  41. Conservation of Mechanical Energy In the absence of friction, total mechanical energy remains the same.

  42. Mechanical Energy Mechanical Energy is not conserved in the presence of friction. As a sanding block slides on a piece of wood, energy (in the form of heat) is dissipated into the block and surface.

  43. Sample Problem Conservation of Mechanical Energy Starting from rest, a child zooms down a frictionless slide from an initial height of 3.00 m. What is her speed at the bottom of the slide? Assume she has a mass of 25.0 kg.

  44. Sample Problem continued Conservation of Mechanical Energy Given: h = hi = 3.00 m m = 25.0 kg vi = 0.0 m/s hf = 0 m Unknown: vf = ?

  45. Sample Problem continued Conservation of Mechanical Energy Choose an equation or situation: The slide is frictionless, so mechanical energy is conserved. Kinetic energy and gravitational potential energy are the only forms of energy present.

  46. Sample Problem continued Conservation of Mechanical Energy The zero level chosen for gravitational potential energy is the bottom of the slide. Because the child ends at the zero level, the final gravitational potential energy is zero. PEg,f= 0

  47. Sample Problem continued Conservation of Mechanical Energy The initial gravitational potential energy at the top of the slide is PEg,i = mghi = mgh Because the child starts at rest, the initial kinetic energy at the top is zero. KEi = 0 Therefore, the final kinetic energy is as follows:

  48. Sample Problem continued Conservation of Mechanical Energy Substitute values into the equations: PEg,i = (25.0 kg)(9.81 m/s2)(3.00 m) = 736 J KEf = (1/2)(25.0 kg)vf2 Now use the calculated quantities to evaluate the final velocity. MEi = MEf PEi + KEi = PEf + KEf 736 J + 0 J = 0 J + (0.500)(25.0 kg)vf2 vf = 7.67 m/s

  49. White Board Physics 1. A HTHS Track Star leaps over a hurdle. If the runner’s initial vertical speed is 2.2 m/s, how much will the runner’s center of mass be raised during the jump?

  50. Practice Problems A pendulum bob is released from some initial height such that the speed of the bob at the bottom of the swing is 1.9 m/s. What is the initial height of the bob? Old Faithful geyser in Yellowstone National Park shoots water every hour to a height of 40m. With what velocity does the water leave the ground? (Assume no air resistance) A 755 N diver drops from a board 10.0 m above the water’s surface. Find the diver’s speed 5.00 m above the water’s surface. Then find the diver’s speed just before striking the water.

More Related