Work & Energy. Chapter 4 pg

Transcription

1 Work & Energy Chapter 4 pg

2 Today s Learning Objectives 1) Know the vocabulary of this chapter. 2) What is the two-pronged test to see if something qualifies as work? 3) Solve and calculate problems about work. 4) Solve and calculate problems about power. 5) What are the advantages of a machine?

3 Work and Power Work When a force causes an object to move in the direction of the force Two prong test to see if it is work: 1. The object must move as the force is applied 2. The direction of the motion is the same as the direction in which the force is applied

4 Work Work or not? Pushing a sled? Picking up a bag of groceries? Carrying your backpack to class? Trying to lift a car with your pinky?

5 Work Formula for work: Work = Force x distance W Fd Units = Joule (J) 1 J = 1 N m

6 MathBREAK A man applies a force of 500 N to push a truck 100 m down the street. How much work does he do? Answer: 50,000 J

7 Power Power The rate at which work is being done Power is how fast work happens Formula: Unit = watt (W) 1 W = 1 J/s Power Work time P W t

8 MathBREAK Which is more powerful, a motor that can do 4500 J of work in 25 seconds, or a motor that can do 2000 J of work in 15 seconds? Answer: 180 W vs. 133 W, so the first one is more powerful

9 Machines (4-1b) Machine Device that changes the size or direction of a force, or motion of work Ex. scissors, screw, bottle opener

10 Machines Advantages of a machine Decrease the force required from your body or Increase the speed and distance of your force or Change the direction of your force

11 Machines Example 1: (decrease the force put in) You put in less force, but you must lift it farther.

12 Machines Example 2: (changing speed) A golf club multiplies the amount of speed you put in.

13 Machines Example 3: (changing direction of force) In a simple pulley like this, the force is the unchanged, but the direction of that force changes.

14 Today s Learning Objectives 1. Know that work output can never be greater than work input. 2. Solve problems (calculate) for mechanical advantage and efficiency.

15 Machines Do machines increase the amount of work done? No. Work that comes out of a machine can NEVER be greater than work that goes in Some of the work you put in is always used to overcome the friction in the machine Work input Work output Work friction Machines can increase your force though, and that makes work feel easier

16 Machines Two ends of a machine Input What goes into the machine (work or force) Output What comes out of a machine (work or force)

17 Machines Mechanical advantage Tells you how many times the machine multiplies force Formula: Mechanical Advantage( MA) Result may be >, =, or < 1 output force input force

18 MathBREAK You apply 200 N to a machine and the machine applies 2600 N to an object. What is the mechanical advantage? Answer: 13 Note, no units required on Mech. Adv.

19 Machines Mechanical Efficiency A comparison of a machine s work output with the work input (always less than 100%) Work output is always less than work input because of friction in the machine Effect lessened with oil Formula: Mechanicalefficiency work output 100 work input

20 Math Break You do 20 J of work pushing a crate up a ramp. If the output work from the ramp is 11 J, then what is the efficiency of the inclined plane? 55%

21 Today s Learning Objectives 1. Be able to draw and label examples of the six types of simple machines. 2. Be able to label diagrams of the three classes of levers.

22 Simple Machines (4-1c) Six simple machines 1. Lever 2. Inclined plane 3. Wedge 4. Screw 5. Wheel and axle 6. Pulley

23 Levers Lever simple machine consisting of a bar that pivots at a fixed point, called a fulcrum 3 kinds of levers: First class Second class Third class

24 First Class Lever First Class Lever Fulcrum between input force and load Can increase or decrease input force, depending on location of fulcrum Input Force

25 Second Class Lever Second Class Lever Load is between the fulcrum and the input force Decrease amount of input force required Ex. wheelbarrow Input Force

26 Third Class Lever Third Class Lever Input force is between fulcrum and load Increases amount of input force required Ex. human forearm Input Force

27 Inclined Plane Inclined Plane Simple machine that is a straight slanted surface Ex. ramp

28 Wedges Wedge A double inclined plane that moves Ex. knife

29 Screws Screw An inclined plane that is wrapped into a spiral (Click: Big Bang quote)

30 Wheel and Axle Wheel and axle Simple machine consisting of two circular objects of different sizes Ex. doorknob, volume knob

31 Pulleys Pulley Simple machine consisting of a grooved wheel that holds a rope or cable Load on one end, input force on other Ex. used in blinds

32 Compound Machines Compound machines Machines made of 2 or more simple machines Most machines fit this category Ex. scissors, zippers, etc. Raising Moai video Raising Obelisk video

33 Today s Learning Objectives What is the difference between kinetic energy and potential energy? How can you calculate kinetic energy? How can you calculate gravitational potential energy? What is the Law of Conservation of Energy? What is Mechanical Energy? Why is mechanical energy NOT always conserved?

34 Energy The ability to do work Describing Energy (4-2) Energy, like work, is measured in Joules (J)

35 Describing Energy Two types of energy 1. Kinetic energy 2. Potential energy

36 Describing Energy Kinetic Energy (KE) The energy of moving objects Ex. A moving car has kinetic energy, a parked car does not. Other examples?

37 Describing Energy Formula: KE = 1 2 mv2 Where m=mass, v=velocity (speed) This indicates the amount of kinetic energy depends on mass and speed of object

38 MathBREAK What is the kinetic energy of a 4000 kg elephant running at 3 m/s? Answer = 18,000 J

39 Potential energy (PE) Describing Energy The energy that is stored due to the interactions between objects (stored energy) Ex. elastic potential energy PE stored in an object when it is stretched or compressed Ex. chemical potential energy PE stored in chemical bonds between atoms

40 Describing Energy Ex. gravitational potential energy the potential energy an object has because it could fall Formula: Where m=mass GPE = mgh g=9.80m/s 2 on Earth h=height

41 MathBREAK If you lift a 5 kg watermelon to the top of a 2 m refrigerator on Earth (where gravity = 9.80 m/s 2 ), how much gravitational potential energy do you give the watermelon? Answer = 98 J

42 Six forms of energy Describing Energy 1. Thermal (KE associated with heat) 2. Chemical (PE stored in chemical bonds) 3. Electrical (KE in moving electrons) 4. Sound (KE in vibrations) 5. Light (KE in photons) 6. Nuclear (PE/KE associated with nuclear forces)

43 Conservation of Energy (4-3) Law of Conservation of Energy Energy cannot be created or destroyed. It can only be converted into other forms, or transferred to other places.

44 Energy Conversion Conservation of Energy A change from one form of energy to another Ex. 2 L bottles, baking soda, & corks 1. Chemical energy of baking soda and vinegar 2. Potential & Thermal energy of gas in bottle 3. Kinetic and sound energy of shooting cork

45 Conservation of Energy Ex. pendulum converting PE to KE Pos. 1 All gravitational potential energy Pos. 2 All kinetic energy Pos. 3 All gravitational potential energy again

46 Conservation of Energy Example 3 Roller Coasters Roller coaster car pulled up a hill, converting KE to PE Car rolls down track, converting PE to KE Car rolls back up hills or loops, converting KE to PE again

47 Conservation of Energy Example 4: Skate Park Simulation

48 Conservation of Energy Ex. 5 electrical energy conversions Many examples Space Heater: Electricity to light / heat Wind Turbine: Kinetic to electrical Battery: Chemical to electrical Electric fan: Electrical to kinetic

49 Conservation of Energy Mechanical energy the sum of KE and PE Even though the Law of Conservation of energy states that energy cannot be created or destroyed, Mechanical energy doesn t stay constant Some of the energy is lost to other forms, or transferred out of the objects being studied. Example: Wind Turbine: Kinetic to electrical, but some energy lost to friction, waste heat, etc.