1 / 53

Work and Energy

Work and Energy. Chapter 12. Students Will Be Able To:. Explain the meaning of work. Explain how work and energy are related. Calculate work. Calculate power. Work.

razi
Download Presentation

Work and Energy

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. Work and Energy Chapter 12

  2. Students Will Be Able To: • Explain the meaning of work. • Explain how work and energy are related. • Calculate work. • Calculate power.

  3. Work • It is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force • It is only done when a force causes a change in the position or the motion of an object in the same direction of the applied force • Therefore, for work to occur, an object must move

  4. Work • Can be calculated by the following equation • Work = force x distance (W= Fd) • W= work • F= force • d= distance

  5. Work • Units for work include • N • m • J • Stands for joule • Is the main unit used for work • kg • m2/s2 • 1J = 1 N • m= 1kg • m2/s2

  6. Example- Work • Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? W = Fd = 190N x 2.0m = 380 N • m = 380 J

  7. Power • It is a quantity that measures the rate at which work is done or energy is transformed • Can be calculated by the following equation • Power = work/time (P= W/t) • P= power • W= work • t= time

  8. Power • Units for power include • J/s • W • Stands for watt • Is the main unit used for power • 1W= 1J/s

  9. Example- Power • It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? • 100kJ x 1000J/1kJ = 100000 J • P = W/t = 100000 J / 20 s = 50000 J/s = 50000 W

  10. Students Will Be Able To: • Explain how machines make work easier. • Explain how to calculate mechanical advantage. • Define efficiency. • Describe the six types of simple machines.

  11. Machines • Machines multiply and redirect forces • Help do work by redistributing the work that is put into them • Can also change the direction of the input force • Able to increase or decrease force by changing the distance over which the force is applied • Different forces can do the same amount of work • Due to decreasing the distance while increasing the force OR decreasing the force while increasing the distance

  12. Mechanical Advantage • Mechanical advantage tells how much a machine multiplies force or increases distance • Calculated by the following equation • Input force is the force applied to the machine • Output force is the force applied by the machine to overcome resistance

  13. Mechanical Advantage • Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine • Some energy transferred is changed to heat due to friction • An ideal machine with no friction would have the same input work and output work

  14. Example- Mechanical Advantage • Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. • Input distance= 5.0 m and output distance= 1.5 m • Mechanical advantage = 5.0m/ 1.5m = 3.33

  15. Efficiency • Is a measure of how much of the work put into a machine is changed into useful output work by the machine • Calculating efficiency • Efficiency = output work/input work X100% • Efficiency of a machine is always less than 100% • Lubricants can make machines more efficient by reducing friction

  16. Example- Efficiency • A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage. • Efficiency = output work/input work X100% = 140 J/ 180 J X 100% = 77.8%

  17. Simple Machines • One of the six basic types of machines, which are the basis for all other forms of machines • Divided into two families • Lever family • Simple lever • Pulley • Wheel and axle • Inclined plane family • Simple inclined place • Wedge • Screw

  18. Levers • Have a rigid arm that turns around a point called the fulcrum • Input arm is the part of the lever on which effort force is applied • Output arm is the part of the lever that exerts the resistance force • Divided into three classes based on the location of the fulcrum and of the input and output forces

  19. First Class Levers • Most common type • Fulcrum is located between the effort and resistance forces (input and output forces) • Multiplies and changes the direction of the force • Example- claw hammer, pliers

  20. Second Class Levers • Fulcrum is located at one end of the arm and the input force (effort force) is applied to the other end • Always multiplies force • Example- wheelbarrow, nutcrackers, hinged doors

  21. Third Class Levers • Effort force is between the output force (resistance force) and fulcrum • Responsible for multiplying distance rather than force • They have a mechanical advantage of less than 1 • Example- human forearm

  22. Pulleys • Are modified levers • The point in the middle of a pulley is like the fulcrum of a lever • A single, fixed pulley has a mechanical advantage of 1 • It is attached to something that doesn’t move • Force is not multiplied but direction is changed • A moveable pulley has one end of the rope fixed and the wheel free to move • It multiplies force • Multiple pulleys are sometimes put together in a single unit called a block and tackle

  23. Wheel and Axles • Is a machine with two wheels of different sizes rotating together • Is a lever or pulley connected to a shaft • The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines

  24. Inclined Planes • Is a sloping surface that reduces the amount of force required to do work • Multiplies and redirects force • Turns a small input force into a large output force by spreading the work out over a large distance • Less force is required if a ramp is longer and less steep

  25. Wedge • Is a modified inclined plane • Functions like two inclined planes back to back • Turns a single downward force into two forces directed out to the sides • Example- nails

  26. Screw • Is an inclined plane wrapped around a cylinder • Example- spiral staircases, jar lids

  27. Compound Machines • Is a machine made of more than one simple machine • Examples of compound machines • scissors, which use two first class levers joined at a common fulcrum • a car jack, which uses a lever in combination with a large screw

  28. Students Will Be Able To: • Explain the meaning of energy. • Distinguish between kinetic and potential energy. • Recognize the different ways that energy can be stored. • Calculate kinetic and potential energy.

  29. Energy • Energy is the ability to do work or to cause change • When you do work on an object, you transfer energy to that particular object • Whenever work is done, energy is transformed or transferred to another system • Energy is measured in joules • Work is also measured in joules

  30. Potential Energy • Is the energy that an object has because of the position, shape, or condition of the object • Is stored energy • 3 types exist • Chemical • Energy stored in chemical bonds between atoms • Elastic • Energy stored in stretched or compressed elastic materials • Gravitational • Energy stored in the gravitational field which exists between two or more objects

  31. Gravitational Potential Energy • Depends on mass, height, and acceleration due to gravity • Can be calculated by the following equation • PE = mass x free fall acceleration x height (PE = mgh) • PE= gravitational potential energy • m= mass • g= free fall acceleration (9.8 m/s2) • h= height • Usually measured from the ground but can also be measured from two objects

  32. Example- Gravitational PE • A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff? • PE = mgh = 65 kg x 9.8 m/s2 x 35 m = 22295 kg•m2/s2 = 22295 J

  33. Kinetic Energy • Is the energy of a moving object due to the object’s motion • Depends on mass and speed • Depends on speed more than mass • Can be calculated by the following equation • Kinetic Energy = ½ mass x speed squared KE = ½ mv2

  34. Example- KE • What is the kinetic energy of a 44 kg cheetah running at 31 m/s? • KE = ½ mv2 = ½ (44 kg)(31m/s)2 = 21142 kg•m/s2 = 21142 J

  35. Other Forms of Energy • The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy • Mechanical energy is the sum of the potential energy and the kinetic energy in a system • In addition to mechanical energy, most systems contain non-mechanical energy • Non-mechanical energy does not usually affect systems on a large scale

  36. Other Forms of Energy • Atoms and molecules have kinetic energy • The kinetic energy of particles is related to heat and temperature • Chemical reactions involve potential energy • The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains • Living things get energy from the sun • Plants use photosynthesis to turn the energy in sunlight into chemical energy

  37. Other Forms of Energy • The sun gets energy from nuclear reactions • The sun is fueled by nuclear fusion reactions in its core • Electricity is a form of energy • Electrical energy is derived from the flow of charged particles, as in a bolt of lightning or in a wire • Light can carry energy across empty space • Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves

  38. Energy Transformations • Energy readily changes from one form to another • Potential energy can become kinetic energy • As a car goes down a hill on a roller coaster, potential energy changes to kinetic energy • Kinetic energy can become potential energy • The kinetic energy a car has at the bottom of a hill can do work to carry the car up another hill

  39. Energy Transformations • Mechanical energy can change to other forms of energy • Mechanical energy can change to non-mechanical energy as a result of friction, air resistance, or other means

  40. Students Will Be Able To: • Describe how energy is conserved when changing from one form to another. • Apply the law of conservation of energy to familiar situations.

  41. The Law of Conservation of Energy • Energy may change from one form to another, but the total amount of energy never changes • Energy doesn’t appear out of nowhere • Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source • Energy doesn’t disappear,but it can be changed to another form

  42. Energy Systems • Scientist study energy systems • Boundaries define a system • Systems may be open or closed • When the flow of energy into and out of a system is small enough that it can be ignored, the system is called a closed system • Most systems are open systems, which exchange energy with the space that surrounds them

  43. Mass  Energy • Mass as thought of as energy when discussing nuclear reactions • The total amount of mass and energy is conserved • 2 processes involved • Nuclear fusion • Two nuclei are fused together • Nuclear fission • Two nuclei are broken apart

More Related