Work and Energy

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

6. 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

7. 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

8. 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

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

10. 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

11. 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.

12. 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

14. 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

15. 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

17. 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

18. 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

19. 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%

20. 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

21. 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

22. 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

23. 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

24. 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

26. 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

28. 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

29. 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

30. 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

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

32. 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

33. 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.

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

45. 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

46. 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.

47. 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

49. 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

50. 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