WORK AND ENERGY

3 WORK Work is done when an object is moved through a distance. It is defined as the product of the component of force applied along the direction of displacement and the magnitude of the displacement. W = F d cos 

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5 Units of Work Look at the equation: W = F d cos , the units are the units of force times displacement. So, Newton meter A Newton meter is called a Joule 1 Joule = 1 Newton meter 1 J = 1 N m

6 Units of Work SI or MKSNewtonmeterJoule CGSDynecentimeterErg EnglishPoundfootftlb The unit is named after the English physicist James P. Joule

7 James Joule ( ) Not formally trained as a physicist. He ran experiments during his honeymoon. He later found the mechanical equivalent of heat. Heat is energy not a substance.

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9 Work: positive or negative? Work is a scalar quantity Work is positive when the component of force and the displacement are in the same direction. Work is negative when the component of force and the displacement are in the opposite directions.

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12 Displacement Force Area under Curve is Work

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Can you determine the work done? 14

15 Who’s Doing Work? AB CD

16 Try this problem A horse pulls a stationary 300 kg cart for 55 m. The horse pulls on the cart at an angle of 35 o with a 450 N constant force. –Find the force along the direction of motion. –Find the work done by the horse. –Find the acceleration of the cart. –Find the total time this takes. –Find the weight of the cart. –What would the d-t, v-t, a-t, F-d graphs look like?

17 POWER ! Power is the rate of doing work. Power is measured in units called Watts

18 Units of Power The units of power are:

The Inclined Plane A Simple Machine 19

20 Independence of Path mg Force applied Length of ramp Work is the same Height

Forms of Energy 21

22 Chemical Energy

23 Solar Energy

24 Electrical Energy

25 Wind Energy

26 Sound Energy

27 Nuclear Energy

28 Heat Energy

29 Kinetic Energy Kinetic Energy is energy of motion.

30 Work-Energy Theorem The result of work is that there is a change of speed, or an acceleration. W=FdW=mad v f 2 = v i 2 + 2ad and solve for ad ad = (v f 2 – v i 2 )/2 Insert into W=mad W=m(v f 2 – v i 2 )/2

31 Work-Energy Theorem

32 Work-Energy Theorem

33 Gravitational Potential Energy The force of gravity does work. It accelerates objects. W = F g d W = mg d W = mg h (h = height above a reference point)

34 Gravitational Potential Energy Note: AP Physics uses the symbol U for potential energy

35 Potential Energy and Work When work is done by gravity or against it, the change in PE is equal to the work done.

Work and Gravity 36

Work and Friction 37

38 Conservation of Energy Energy is conserved in all systems. This means that in the Universe, all the energy that was at the beginning of time is still in the Universe today. As we isolate systems, we find that the total energy before is equal to the total energy after. The energy just changes form.

39 Mechanical Energy

40 Try this Problem Find the final speed of the object as it hits the pavement. 35m

The Roller Coaster 41

Conservation of Energy- The Roller Coaster 42

43 The Roller Coaster Problem 40 m 20 m 30 m A C B E D F Find the Speeds at points A-F 25 m

Atwood’s Machine Revisited 44

45 Spring Potential Energy Springs possess potential energy if they are elongated or compressed from their rest positions.Springs possess potential energy if they are elongated or compressed from their rest positions. This is called spring (or elastic) potential energy.This is called spring (or elastic) potential energy. The formula is derived from Hooke’s Law:The formula is derived from Hooke’s Law:

46 Spring Potential Energy

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50 Conservative Force A conservative force is such that the work done on the object is independent of the path. OR The force does NO work as the object moves around a closed path from start to finish.

51 Conservative vs. Non-conservative Force Conservative –Gravity –Elastic Spring –Electric Non-conservative –Friction –Air resistance –Tension –Normal force

Conservative Force 52

Non-Conservative Force 53

54 Internal Energy The total energy contained within a thermodynamic system. Basically we are talking about heat when we use the term internal energy Usually in mechanical systems, heat is created because of friction, air resistance, or water resistance Energy in this form is typically wasted energy

55 Non-Conservative Forces A force is non-conservative if the work done on an object is dependent upon the path of travel.

56 Review Questions Write responses to each question on a separate sheet of paper to be handed in. Write responses to each question on a separate sheet of paper to be handed in. There are 6 questions. There are 6 questions. Some are equations, some require written responses and deep thought, while some are graphical. Some are equations, some require written responses and deep thought, while some are graphical.

57 Question 1 What is the definition of Work in Physics?

58 Question 2 Name two ways in which Work and Energy are related.

59 Question 3 What equation equates Work with Kinetic Energy?

60 Question 4 Explain how this picture shows that:

61 Question 5 How does the statement, “the Earth gets all its energy from the Sun,” explain how you can be awake here today?

62 Question 6 Draw a graph that shows how PE and KE change while riding this roller coaster. Label points W, X, and Y on the graph.

In Summary 63 Work is Force times a displacement. Kinetic Energy is 1/2 mv 2. Gravitational Potential Energy is mg  h. The work-energy theorem is W=  K. Conservation of energy Spring potential energy is 1/2 kx 2