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Welcome back to Physics 215

Welcome back to Physics 215. Today ’ s agenda: Work Non-conservative forces Power. Current homework assignment. HW7: Knight Textbook Ch.9: 54, 72 Ch.10: 48, 68, 76 Ch.11: 50, 64 Due Wednesday, Oct. 22 nd in recitation. Work, Energy. Newton ’ s Laws are vecto r equations

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Welcome back to Physics 215

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  1. Welcome back to Physics 215 Today’s agenda: Work Non-conservative forces Power

  2. Current homework assignment • HW7: • Knight Textbook Ch.9: 54, 72 • Ch.10: 48, 68, 76 • Ch.11: 50, 64 • Due Wednesday, Oct. 22nd in recitation

  3. Work, Energy • Newton’s Laws are vector equations • Sometimes easier to relate speed of a particle to how far it moves under a force – a single equation can be used – need to introduce concept of work

  4. What is work? • Assume constant force in 1D • Consider: vF2 = vI2 + 2as • Multiply by m/2  (1/2)mvF2 - (1/2)mvI2 = mas • But: F = ma  (1/2)mvF2 - (1/2)mvI2 = Fs

  5. Work-Kinetic Energy theorem (1) (1/2)mvF2 - (1/2)mvI2 = Fs Points: • W = Fs  defineswork done on particle = force times displacement • K = (1/2)mv2  defineskinetic energy =1/2 mass times square of v

  6. Work-Kinetic Energy demo • Cart, force probe, and motion detector • Plot v2 vs. x – gradient 2F/m • constant F (measure) -- pulling with string • Weigh cart and masses in advance

  7. Conclusions from experiment Although the motion of the two carts looks very different (i.e., different amounts of time, accelerations, and final speeds), there is a quantity that is the same for both at the end of the motion. It is (1/2) mv2 and is called the (final) kinetic energy of the carts. Moreover, this quantity happens to have the same value as F Ds, which is given the name work.

  8. Improved definition of work • For forces, write F  FAB • Thus W = Fs  WAB = FAB DsA is work done on A by B as A undergoes displacement DsA • Work-kinetic energy theorem: Wnet,A = SBWAB = DK

  9. Non-constant force … Work done in small interval Dx DW = F Dx Total W done from A to B  SF Dx = Area under curve! F F(x) x B A Dx

  10. The Work - Kinetic Energy Theorem Wnet = DK = Kf - Ki The net work done on an object is equal to the change in kinetic energy of the object.

  11. Suppose a tennis ball and a bowling ball are rolling toward you. The tennis ball is moving much faster, but both have the same momentum (mv), and you exert the same force to stop each. Which of the following statements is correct? 1. It takes equal distances to stop each ball. 2. It takes equal time intervals to stop each ball. 3. Both of the above. 4. Neither of the above.

  12. Suppose a tennis ball and a bowling ball are rolling toward you. Both have the same momentum (mv), and you exert the same force to stop each. The distance taken for the bowling ball to stop is the distance taken for the tennis ball to stop. 1. less than. 2. equal to 3. greater than

  13. Two carts of different mass are accelerated from rest on a low-friction track by the same force for the same time interval. Cart B has greater mass than cart A (mB > mA). The final speed of cart A is greater than that of cart B (vA > vB). After the force has stopped acting on the carts, the kinetic energy of cart B is 1. less than the kinetic energy of cart A (KB < KA). 2. equal to the kinetic energy of cart A (KB = KA). 3. greater than the kinetic energy of cart A (KB > KA). 4. “Can’t tell.”

  14. Revised definitions for Work and Kinetic Energy Work done on object 1 by object 2: Kinetic energy of an object:

  15. Scalar (or “dot”) product of vectors The scalar product is a way to combine two vectors to obtain a number (or scalar). It is indicated by a dot (•) between the two vectors. (Note: component of A in direction n is just A•n)

  16. Is the scalar (“dot”) product of the two vectors 1. positive 2. negative 3. equal to zero 4. “Can’t tell.”

  17. Is the scalar (“dot”) product of the two vectors 1. positive 2. negative 3. equal to zero 4. “Can’t tell.”

  18. Two identical blocks slide down two frictionless ramps. Both blocks start from the same height, but block A is on a steeper incline than block B. The speed of block A at the bottom of its ramp is 1. less than the speed of block B. 2. equal to the speed of block B. 3. greater than the speed of block B. 4. “Can’t tell.”

  19. Solution • Which forces do work on block? • Which, if any, are constant? • What is F•Ds for motion?

  20. Work done by gravity Work W = -mg j•Ds Therefore, W = -mgDh N does no work! N Ds mg j i

  21. Work done on an object by gravity W(on object by earth) = – m gDh, where Dh = hfinal – hinitial is the change in height.

  22. Defining gravitational potential energy The change in gravitational potential energy of the object-earth system is just another name for the negative value of the work done on an object by the earth.

  23. Curved ramp s = W = F•s = Work done by gravity between 2 fixed pts does not depend on path taken! Work done by gravitational force in moving some object along any path is independent of the path depending only on the change in vertical height

  24. Hot wheels demo One hot wheels car, car A, rolls down the incline and travels straight ahead, while the other car, B, goes through a loop at the bottom of the incline. When the cars reach the end of their respective tracks, the relative speeds will be: vA > vB vA < vB vA = vB Can’t tell

  25. A person carries a book horizontally at constant speed. The work done on the book by the person’s hand is 1. positive 2. negative 3. equal to zero 4. “Can’t tell.”

  26. A person lifts a book at constant speed. Since the force exerted on the book by the person’s hand is in the same direction as the displacement of the book, the work (W1) done on the book by the person’s hand is positive. The work done on the book by the earth is: 1. negative and equal in absolute value to W1 2. negative and less in absolute value than W1 3. positive and equal in absolute value to W1 4. positive and less in absolute value than W1

  27. A person lifts a book at constant speed. The work (W1) done on the book by the person’s hand is positive. Work done on the book by the earth: Net work done on the book: Change in kinetic energy of the book:

  28. Work-Kinetic Energy theorem(final statement) W1net =Fnet •Ds1 = Kf - Ki

  29. A person lifts two books (each of mass m) at constant speed. The work done on the upper book by the lower book is positive. Its magnitude is W = m g Ds. Is there work done on the lower bookby the upper book? 1. Yes, positive work. 2. Yes, negative work. 3. No, zero work. 4. No, since the lower book does work on the upper book, this is not a meaningful question.

  30. A person lifts two books at constant speed. The work done on the upper book by the lower book is positive. Work done on lower book by upper book: Net work done on the lower book: Change in kinetic energy of the lower book:

  31. Conservative forces • If the work done by some force (e.g. gravity) does not depend on path the force is called conservative. • Then gravitational potential energy Ug only depends on (vertical) position of object Ug = Ug(h) • Elastic forces also conservative – elastic potential energy U = (1/2)kx2...

  32. Nonconservative forces • friction, air resistance,... • Potential energies can only be defined for conservative forces

  33. Nonconservative forces • Can do work, but cannot be represented by a potential energy function • Total mechanical energy can now change due to work done by nonconservative force • For example, frictional force leads to decrease of total mechanical energy -- energy converted to heat, or internal energy • Total energy = total mech. energy + internal energy -- is conserved

  34. Conservation of (mechanical) energy • If we are dealing with a potential energy corresponding to a conservative force 0 = DK+DU • Or K + U = constant

  35. Conservation of total energy The total energy of an object or system is said to be conserved if the sum of all energies (including those outside of mechanics that have not yet been discussed) never changes. This is believed always to be true.

  36. Demo Vertical spring and mass with damping • Damping from air resistance • Amplitude of oscillations decays • Oscillations still about x = -xeq

  37. Many forces • For a particle which is subject to several (conservative) forces F1, F2 … E = (1/2)mv2 + U1 + U2 +… is constant • Principle called Conservation of total mechanical energy

  38. Summary • Total (mechanical) energy of an isolated system is constant in time. • Must be no non-conservative forces • Must sum over all conservative forces

  39. A compressed spring fires a ping pong ball vertically upward. If the spring is compressed by 1 cm initially the ball reaches a height of 2 m above the spring. What height would the ball reach if the spring were compressed by just 0.5 cm? (neglect air resistance) • 2 m • 1 m • 0.5 m • we do not have sufficient information to calculate the new height

  40. A 5.00-kg package slides 1.50 m down a long ramp that is inclined at 12.0 below the horizontal. The coefficient of kinetic friction between the package and the ramp is mk = 0.310. Calculate: (a) the work done on the package by friction; (b) the work done on the package by gravity; (c) the work done on the package by the normal force; (d) the total work done on the package. (e) If the package has a speed of 2.20 m/s at the top of the ramp, what is its speed after sliding 1.50 m down the ramp?

  41. Summary • Work is defined as dot product of force with displacement vector • Each individual force on an object gives rise to work done • The kinetic energy only changes if net work is done on the object, which requires a net force

  42. Power Power = Rate at which work is done Units of power:

  43. A sports car accelerates from zero to 30 mph in 1.5 s. How long does it take to accelerate from zero to 60 mph, assuming the power (=DW / Dt) of the engine to be constant? (Neglect losses due to friction and air drag.) 1. 2.25 s 2. 3.0 s 3. 4.5 s 4. 6.0 s

  44. Power in terms of force and velocity

  45. A locomotive accelerates a train from rest to a final speed of 40 mph by delivering constant power. If we assume that there are no losses due to air drag or friction, the acceleration of the train (while it is speeding up) is 1. decreasing 2. constant 3. increasing

  46. A ball is whirled around a horizontal circle at constant speed. If air drag forces can be neglected, the power expended by the hand is: 1. positive 2. negative 3. zero 4. “Can’t tell.”

  47. Reading assignment • Extended objects, center of mass • Begin Chapter 12 in textbook

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