Chapter 12 - Work and Energy. Section 1 - Work, Power, and Machines

Transcription

1 Chapter 12 - Work and Energy Section 1 - Work, Power, and Machines 1

2 Imagine trying to lift a car without a jack You might be exerting a lot of force, but not moving the It would feel like you have done a lot of work...but you have actually done NONE 2

3 In science, is done only when a force causes a change in the position or motion of an object work = force x distance So what are the units we measure work in? 3

4 Work is measured in a unit called 1 J = 1 Nm = 1 kgm Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each 4

5 Imagine walking up a flight of stairs vs. running up a Which is more exhausting? 5

6 The amount of time it takes to do work is an important factor measures work in relation to time power = work/time 6

7 So what are our units for power? Joules/second = Watt watt (W) is the amount of power required to do 1 J of This is about how much power you would need to lift an apple over your head in 1 second 746 watts = 1 horsepower 7

8 It takes 100kJ of work to lift an elevator 18 m. If this is done in 20 seconds, what is the average power of the elevator during this process? 8

9 Which is easier...lifting a car with a jack, or by yourself? 9

10 Compare the amount of work required to lift a fourwheeler straight up onto the bed of a pickup, with the amount of work required to push the same four- The ramp allows you to apply a smaller larger distance, but the work done is the same 10

11 allow us to do the same amount of work by the distance while force or by 11

12 a machine describes how mechanical advantage = input force input distance 12

13 A machine with a mechanical advantage greater than 1 multiplies the input force force, but increases distance and speed 13

14 Calculate the mechanical advantage of a ramp that is 5.0 meters long and 1.5 meters high 14

15 Section 2 - Simple Machines 15

16 16

17 The Lever Family Levers are divided into three classes: 1st, 2nd, and 3rd Class All levers have a rigid that pivots around a point called the fulcrum 17

18 have a fulcrum located between the input and output forces Examples: Seesaw, hammer 18

19 the arm and the input force is applied at the other Examples: Wheelbarrow, two wheel cart (dolly) 19

20 Third-Class Levers have a fulcrum at one end with the output force on the other end. They multiply distance rather than force. Therefore have a mechanical advantage of less than 1 20

21 Pulleys are modified levers because the pulley itself is like the fulcrum of a lever A single fixed pully has a mechanical advantage 21

22 A single movable pulley has a mechanical advantage of 2 Therefore you can lift a 100 pounds with only exerting 22

23 Multiple pulleys can be used to get even higher mechanical advantages When multiple pulleys are put together in a single unit it is called a 23

24 wheel and axle is a lever or pulley connected to a shaft A small input force can be applied and the force is multiplied to become a large output force on the shaft Steering wheel, screwdrivers, cranks 24

25 The Inclined Plane Family In inclined plane is basically a ramp The input force is parallel to the ramp object off the ground Output Force 25

26 Remember, incline planes work by spreading the work out over a larger distance, which decreases the force required! 26

27 is a modified inclined plane A wedge is two incline planes placed back to back The wedge turns a single downward force into two forces directed out to the sides Examples: Axe, nail 27

28 is an inclined plane wrapped around a cylinder The steeper the threads, the more force required to Examples: Screws, jar lids, spiral staircase 28

29 machines combines two or more simple A scissor: Uses two first class levers joined at a single fulcrum AND a wedge that cuts into the paper Bicycle: Wheel and axle, levers 29

30 Section 3 - What is Energy? 30

31 Remember, work is only done when an object experiences a change in its position or motion However, energy can be present in an object when is the ability to do work Energy and work are both expressed in units of 31

32 A stretched rubber band (say on a slingshot) has stored energy Potential Energy Often called "energy of position" Think of an apple hanging form a tree...the energy that could potentially do work on the apple results from its position above the ground This is called gravitational potential energy 32

33 The equation to calculate gravitational potential energy is: Potential energy = mass x free fall acceleration x height PE = mgh This means that potential energy is dependent on mass and height 33

34 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? 34

35 of its motion is the energy an object has because starts to fall it gains kinetic energy 35

36 The equation for calculating kinetic energy is: Kinetic energy = 1/2 x mass x speed squared kinetic energy? 36

37 What is the kinetic energy of a 44 kg cheetah running 37

38 The sum of potential energy and kinetic energy is mechanical energy Are there any other forms of energy? We eat it for energy right? 38

39 Energy that does not affect the motion of an object is nonmechanical energy Atoms and molecules have kinetic energy because they are moving on an atomic scale Chemical reactions involve potential energy - Chemical bonds store energy 39

40 The sun gets its energy from nuclear reactions - Nuclear fusion, nuclear fission Electricity is a form of energy lectromagnetic waves carries energy across empty space 40