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What do you think of when you hear these words: Work… Energy… Power…

What do you think of when you hear these words: Work… Energy… Power…. Energy. Universe is made up of matter and energy. Energy is the mover of matter. Energy has several forms: Kinetic Potential Electrical Heat etc. http://tiki.oneworld.net/energy/energy.html. Energy.

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What do you think of when you hear these words: Work… Energy… Power…

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  1. What do you think of when you hear these words: Work… Energy… Power…

  2. Energy • Universe is made up of matter and energy. • Energy is the mover of matter. • Energy has several forms: • Kinetic • Potential • Electrical • Heat • etc. http://tiki.oneworld.net/energy/energy.html

  3. Energy • Conversions from one form of energy to another continually occur. • Energy cannot be created or destroyed.

  4. Work • Force applied in the direction of motion • Work is measured in Joules (J) = Nm • Another way to measure work is in calories • 1 cal = 4.186 J • 1kcal = 1 Calorie = 1000 cal

  5. Work or No Work

  6. In order to do work: • A force must be applied • That force must result in the object’s movement

  7. Work = Force|| x Distance Work = (F cos θ) d Θ is between the force and displacement vectors In this case, the distance is the magnitude of the displacement. Only the component of force parallel to the displacement does work

  8. x

  9. Positive Work • Work done on an object • Force causes motion Fapp = 50 N d = 4 m

  10. Negative Work • Work done by an object • Force opposes motion (Friction always does negative work!) Ff = 10 N d = 4 m

  11. A genius cheerleader named Brandon lifts his 50.0-kg partner straight up off the ground a distance of 0.60 m before releasing her. If he does this 20 times, how much work has he done? • WTot = 5,880 J

  12. “WORK” this out!!!!!!!! • A 0.75 kg block slides with uniform velocity down a 20º inclined plane. A) How much work is done by the force of friction on the block as it slides the total length of the plane? B) What is the net work done on the block? C) Discuss the net work done if the angle of the ramp increased so that the block accelerates down the ramp. Additional Information L = 1.2 m μ = tan 20º

  13. A = -3.2 J • B = Work done by gravity + Work done by friction = Total Work • Since work is scalar add to find Net Work! • C = If it accelerates down then gravitational force is greater than opposing force, so net work is done on the box • (Wg > Wf) Causes a change in the amount of energy an object has!

  14. Do you feel more tired after walking a mile or running a mile? • Why? Do you do the same amount of work? • Same amount of work ( same force over same distance) • Running is the ability to do work at a higher rate! Running requires MORE.....

  15. Power • Rate at which work is done • measured in watts (W) • One watt of power is expended when one joule of work is done in one second. http://www.cartoonstock.com/newscartoons/cartoonists/pto/lowres/pton114l.jpg

  16. Power Power tells you how fast work is being done or how fast energy is transferred! P = Fd cos θ = Fv cos θ t

  17. What is horsepower? • 1 horsepower = 746 Watts • A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute.

  18. Mechanical Energy Are you talking about ME? • Energy possessed by an object due to its motion or its stored energy of position. • It can either be potential energy or kinetic energy. • All forms of energy are measured in joules (J).

  19. Potential Energy • stored energy • …the ability (potential) to do work • An object’s ability to do work by virtue of its position or chemical make-up • Types: elastic, gravitational and chemical. • Examples: • Rubber bands • Springs • Bows • Batteries • Gravity? PE is my favorite subject!

  20. Gravitational Potential Energy • PE = Weight ´ height • PE = m g h • Question: • How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?

  21. Potential Energy • The same work is done on each block.  What mattersis the final elevation, not thepath followed http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html

  22. Kinetic Energy • The energy of motion. • Kinetic Energy = ½ mass ´ velocity2 KE = ½ mv²

  23. Check for Understanding Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s. If the roller coaster car in the last problem were moving with twice the speed, then what would be its new kinetic energy?

  24. Law of Conservation of Energy • Energy cannot be created or destroyed. • Energy can be transformed from one form to another, but the total amount of energy never changes.

  25. Energy is Conserved! It is just changing forms!

  26. More Energy Transfer • High speed winds are used to do work on the blades of a turbine at the so-called wind farm. • Mech. Energy from the air gives the air particles the ability to apply a force to the blades. • As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.

  27. Energy Transfer... • This diagram shows that the potential energy of the boy is changing as he swings. Explain what is happening, in other words how is the energy changing?

  28. Work/Energy Relationship • If you want to move something, you have to do work. • Energy is the ability to do work or cause a change. • The work done is equal to the change in kinetic energy. • W = DKE http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html

  29. A shuffleboard player pushes a 0.25 kg puck, initially at rest, such that a constant horizontal force of 6.0 N acts on it through a distance of 0.50 m. (Neglect friction.) a) What are the kinetic energy and the speed of the puck when the force is removed? B) How much work would be required to bring the puck to rest?

  30. W = Fd = Δ KE • W = 3.0 J • v = 4.9 m/s • B) - 3.0 J

  31. Forces in Car Crashes • http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html#cc3

  32. Seatbelt Physics http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html#cc1

  33. PE = mgh If potential energy at top of path is 100 J, how much kinetic energy does the ball have just before it hits the ground and comes to a stop? mg h KE = ½ mv2 mg v

  34. The formula for KE is _____? • Kinetic Energy is dependent upon ___ & ____ • If an object’s velocity doubles, its kinetic energy _____. If velocity triples, it will have _____ times the kinetic energy. • If mass doubles, kinetic energy _____. If mass triples, kinetic energy _____. • …so what happens to KE if mass doubles and velocity doubles? What if mass is cut in half and velocity doubles?

  35. Think about standing on your desk…if you jumped off, how fast would you be moving just prior to reaching the floor? • How much work would you do on the floor? http://school.discoveryeducation.com/clipart/images/in-desk.gif

  36. Object Falling from Rest http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2

  37. Bungee Jumping • You wish to bungee jump off of a platform. Assuming that there is no air resistance and the spring constant of the 40 m long bungee cord is 100 N/m. How high should your platform be? • Determine all the forces acting ON the body. • Draw a free body diagram. • Apply Newton's second law. • Think about elastic potential energy. • Solve.

  38. A Thought Experiment: • Suppose that you were captured by an evil physicist who gave you the following choice: • You must either: • Stand in front of a 1000 kg. truck moving at 1 m/s, or • Stand in front of a 1 kg. frozen meatball moving at 1000 m/s. • …think…

  39. Truck: • Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s • Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules • Meatball: • Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s • Meatball's kinetic energy = 0.5 mv2 = (0.5)(1 kg)(1000 m/s)2 = 500 000 Joules

  40. Diagramming: • A 120 kg lineman moving west at 2 m/s tackles an 80 kg football fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw a vector diagram in which the before- and after-collision momenta of each player is represented by a momentum vector. Label the magnitude of each momentum vector.

  41. Answer to previous slide question

  42. impulse = momentum

  43. Bouncing increases impulse explain...

  44. Impulse= change in momentum

  45. Impulse • Rhonda, who has a mass of 60.0 kg, is riding at 25.0 m/s in her sports car when she must suddenly slam on the brakes to avoid hitting a dog crossing the road. She strikes the air bag, which brings her body to a stop in 0.400 s. What average force does the seat belt exert on her?

  46. Why wear seatbelts? • If Rhonda (from the previous question) had not been wearing her seat belt and not had an air bag, then the windshield would have stopped her head in 0.001 s. What average force would the windshield have exerted on her?

  47. Elastic collisions: momentum is redistributed or transferred without loss. m1v1i + m2v2i = m1 v1f +m2v2f

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