Chapters 10 & 11: Energy

Transcription

1 Chapters 10 & 11: Energy

2 Power: Sources of Energy

3 Tidal Power SF Bay Tidal Power Project

4 Main Ideas (Encyclopedia of Physics) Energy is an abstract quantity that an object is said to possess. It is not something you can directly observe. The usefulness of the concept comes from the Conservation of Energy. In predicting the behavior of objects, one uses the Conservation of Energy to keep track of the total energy and the transfer of energy between its various forms and between objects. Work is the transfer of energy from one object to another by a force from one on the other that displaces the other. Power is the rate at which energy is transferred or, the rate at which work is done. Power is the FLOW of energy.

5 Conservation of Energy Energy can neither be created nor destroyed. It may change in form or be transferred from one system to another. The total amount of energy in the Universe is constant and can never change. E i E f Except for VERY brief amounts of time according to the Heisenberg Uncertainty Principle.

6 Ways to Transfer Energy Into or Out of A System Work transfers by applying a force and causing a displacement of the point of application of the force Mechanical Waves allow a disturbance to propagate through a medium Heat is driven by a temperature difference between two regions in space

7 More Ways to Transfer Energy Into or Out of A System Matter Transfer matter physically crosses the boundary of the system, carrying energy with it Electrical Transmission transfer is by electric current Electromagnetic Radiation energy is transferred by electromagnetic waves

8 Work Done by a Constant Force A force applied across a distance. W F r

9 Work-Energy Theorem The net work done changes the Kinetic Energy. If the velocity is constant, then the net work is zero. Wnet KE r 1 1 W KE F r ma r mv mv v v 2a r Notice: 2 2 net f i f i

10 Units Energy & Work are scalars and have units of the Joule: m 2 W Fd N m kg J s 2 1 m m 2 KE mv 2 kg( ) 2 kg J 2 s s 2

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12 Kinetic Energy The energy an object has due to its motion. KE 1 2 mv 2 IMPORTANT! v is the TOTAL velocity and is a scalar!!!

13 Kinetic Energy Table 7.1, p.194

14 Work Done by a Constant Force A force applied across a distance. W F r

15 If the Force and displacement are given by vectors use Scalar Product of Two Vectors! The scalar product of two vectors is written as A. B It is also called the dot product A B Ax Bx AyBy Az Bz A. B = A B cos q q is the angle between A and B q AB AB 1 cos ( ) W F r cosq Becomes W F r

16 Constant Force Problem If the resultant force acting on a 2.0-kg object is equal to (3i + 4j) N, what is the change in kinetic energy as the object moves from (7i 8j) m to (11i 5j) m? If the object was initially at rest, what is the final velocity? a. +36 J b. +28 J c. +32 J d. +24 J e. +60 J

17 Gravity and Air Resistance A 12-kg projectile is launched with an initial vertical speed of 20 m/s. It rises to a maximum height of 18 m above the launch point. How much work is done by the dissipative (air) resistive force on the projectile during this ascent? a kj b kj c kj d kj e kj

18 Work Done by a Varying Force The work done is equal to the area under the curve W x x i f F dx x where F x Fcosq

19 Varying Force Problem A force acting on an object moving along the x axis is given by Fx = (14x 3.0x 2 ) N where x is in m. How much work is done by this force as the object moves from x = 1 m to x = +2 m? a. +12 J b. +28 J c. +40 J d. +42 J e. 28 J

20 Work Done by a Varying Force Hooke s Law F s = - kx

21 Hooke s Law Ut tensio, sic vis - as the extension, so is the force Hooke s Law describes the elastic response to an applied force. Elasticity is the property of an object or material which causes it to be restored to its original shape after distortion. An elastic system displaced from equilibrium oscillates in a simple way about its equilibrium position with Simple Harmonic Motion.

22 Elastic Systems F kx Small Vibrations

23 Robert Hooke ( ) Leading figure in Scientific Revolution Contemporary and arch enemy of Newton Hooke s Law of elasticity Worked in Physics, Biology, Meteorology, Paleontology Devised compound microscope Coined the term cell

24 Hooke s Law F s = - kx The Restoring Force When x is positive (spring is stretched), F is negative When x is 0 (at the equilibrium position), F is 0 When x is negative (spring is compressed), F is positive

25 Hooke s Law It takes twice as much force to stretch a spring twice as far. The linear dependence of displacement upon stretching force: Fapplied kx

26 Hooke s Law Stress is directly proportional to strain. F ( stress) kx( strain) applied

27 Spring Constant k: Stiffness The larger k, the stiffer the spring Shorter springs are stiffer springs k strength is inversely proportional to the number of coils

28 Work done by a Spring The horizontal surface on which the block slides is frictionless. The speed of the block before it touches the spring is 6.0 m/s. How fast is the block moving at the instant the spring has been compressed 15 cm? k = 2.0 kn/m The mass of t he block is 2.0 kg. a. 3.7 m/s b. 4.4 m/s c. 4.9 m/s d. 5.4 m/s e. 14 m/s

29 If F is constant then If F varies with r then along any radial segment The total work is Last Time: Work A force moving a mass through a displacement does WORK. W Fr Fr cosq W F r dw F dr F() r dr W r r i f F( r) dr

30 Work Done by a Spring Identify the block as the system The work is the area under the Calculate the work as the block moves from x i = - x max to x f = 0 x f 0 1 Ws Fxdx x kx dx kx i xmax 2 The total work done as the block moves from x max to x max is zero. 2 max

31 Energy in a Spring What speed will a 25g ball be shot out of a toy gun if the spring (spring constant = 50.0N/m) is compressed 0.15m? Ignore friction and the mass of the spring. W Use Energy! spring KE ball 1 1 kx mv 2 2 v 2 2 k x m 50.0 N/ m v (.15 m) 6.7 m / s.025kg

32 Work Done by a Varying Force: Gravity The interplanetary probe is attracted to the sun by a force given by: F 1.3x10 r Sun F Probe mm G r r 1 2 ˆ 2 Find the work done ON the probe by the sun as it moves from an orbital radius of 1.5x10 11 m to 2.3x10 11 m. The negative sign indicates that the force is attractive. This is because of the way that the polar unit vectors are defined. With the origin located at the sun and the radial vector pointing towards the probe, the force of gravity acting on the probe is in the negative direction.

33 Work Done by a Varying Force: Gravity The probe is moving away from the sun so the work done ON the probe BY the sun is slowing it down. Thus, the work should be negative. W 2.3x x x10 2 x 22 dx F 1.3x10 r x x10 ( ) 3x10 10 J 2.3x x Attractive force versus distance for interplanetary probe. The area under the curve is negative since curve is below x-axis.

34 Binding Energy in Molecules

35 Energy States Left to their own devices, systems always seek out the lowest energy state available to them. Systems want to be at rest or in a constant state of motion. You have to do work on the Rock to roll it back up the hill. This will give the Rock Energy the potential of rolling back down Potential Energy.

36 Potential Energy The energy an object has due to its position in a force field. For example: gravity or electricity The Potential Energy is relative to a ground that is defined. (Potential Energy: U or PE)

37 Gravitational Potential Energy PE mgh Force Distance It takes work to move the object and that gives it energy! Same change in height! PE = mgh The ground : h = 0 IMPORTANT! Either path gives the same potential energy! WHY?

38 Work Up an Incline The block of ice weighs 500 Newtons. How much work does it take to push it up the incline compared to lifting it straight up? Ignore friction.

39 Work Up an Incline Work = Force x Distance Straight up: W Fd 500N 3m 1500J 3 F? F mg sinq 500N 250N 6 W Fd 250N 6m 1500J Push up: What is the PE at the top? 1500J mg = 500N An incline is a simple machine!

40 Simple Machines Force Multipliers 1500J Same Work, Different Force, Different Distance

41 ONLY Conservative Forces have Potential Energy Functions!! The work done by a conservative force on a particle moving between any two points is independent of the path taken by the particle. Examples: Gravity, Spring force & Electricity The work done by a conservative force on a particle moving through any closed path is zero A closed path is one in which the beginning and ending points are the same

42 Conservative Forces and Potential Energy A conservative force is defined as a vector field that is equal to the gradient of a scalar field the Potential Energy function U: Define the work done by such a force, F, through a displacement x as F x du dx The gradient of a scalar field at a point is a vector pointing in the direction of the steepest slope or grade at that point. The steepness of the slope at that point is given by the magnitude of the gradient vector. x f WC Fx dx U xi

43 Gravitational Potential Energy with Constant Force The work on the book by lifting it slowly through a vertical displacement The work done on the system must appear as an increase in the energy of the system r yˆj W F app r W ( mgˆj) yf yi W mgy mgy f i ˆj Ug mgy du d( mgy) Check: Fy mg!! dy dy

44 Varying Gravitational Force Potential Energy & Equipotential F mm G r 1 2 rˆ 2 F r du dr Potential Energy Due to Point Masses: Gradients & Equal Potential Energy Surfaces

45 Varying Electric Force Potential Energy & Equipotential F qq k r 1 2rˆ 2 F r du dr Potential Energy Due to Point Charges: Gradients & Equal Potential Energy Surfaces

46 Varying Gravitational Force Potential Energy & Equipotential F mm G r 1 2 rˆ 2 F r du dr Potential Energy Due to Point Masses: Gradients & Equal Potential Energy Surfaces

47 Gravitational Potential Energy for a variable force As a particle moves from A to B, its gravitational potential energy changes by f U U f Ui W F() r dr Choose the zero for the gravitational potential energy where the force is zero: U i = 0 where r GmM E GmM U f U() r dr 2 r r GM Em Ur () r r r i r Systems bound by Attractive Forces have Negative Energy States! Negative because it is a BOUND system. You must put work into the system to move the mass away from the planet. E i

48 Total Mechanical Energy The total mechanical energy of a system is defined as the sum of the kinetic and potential energies: E K U mech If only conservative forces act, the total mechanical energy is conserved. K U K U f f i i U K 0

49 Work Done by a Conservative Force Conservative forces do work on a system such that Energy is exactly transferred between kinetic energy and potential energy, there is no energy transferred (lost) to friction or heat. Thus Conservation of Mechanical Energy gives: E E i f Ki Ui K f U f ( U U ) K K f i f i U K By the Work-Energy Theorem: Wc K The work done by a conservative force: Wc U

50 Conservation of Mechanical Energy If there are no frictional forces, PE is converted into KE. Total Energy: 10,000J Total Energy: 10,000J Total Energy: 10,000J Total Energy: 10,000J Total Energy: 10,000J

51 Conservative Force of Gravity Frictionless Ramp 50J 0J E i E KE PE KE PE i i f f f 25J 25J 0J 50J

52 Conservation of Mechanical Energy Potential Energy of a Spring Kinetic Kinetic & Potential Potential Kinetic

53 Ski Hill Problem If the skier has an initial velocity of 12m/s, what is his final velocity at the top of the ramp? Ignore Friction. 12 m/ s

54 Ski Hill Problem Take the ground to be the ground: U = 0. E E i f Ki Ui K f U f mvi 0 mv f mgh v f vi 2gh 9.75 / 12 m/ s m s PE=0

55 Nonconservative Forces Nonconservative forces do work on a system such that Mechanical Energy is lost or transformed into internal energy (heat) or can t be directly transformed back into KE or PE. NOT ONLY RETARDING FORCES!! Nonconservative forces can speed systems up!!! Conservation of Energy gives: Energy Bank E W nc Expenditures Ki Ui K f U f Wnc Examples : Air resistance, Friction, Applied Forces

56 Nonconservative Forces The change in mechanical energy of a system is due to the nonconservative forces acting on it. E W nc Ki Ui K f U f Wnc U K W nc Ex: Change in energy bank spent on friction: U K fd

57 Ski Hill Problem If the skier has an initial velocity of 12m/s, what is his final velocity at the top of the ramp? The coefficient of kinetic friction between the skies and the hill is The mass of the skier is 80kg. 12 m/ s PE=0

58 Ski Hill Problem Ki Ui K f U f W f mvi 0 mv f mgh fd 2 2 Wf f d v v 2gh 2 fd / m f 2 i What is f and d? m=80kg 12 m/ s d.13 PE=0

59 Ski Hill Problem v v 2gh 2 fd / m f 2 i f mg cos q, d h/sinq v v 2gh 2g cos q ( h/ sin q) f 2 i v v gh q f 2 (1 cot ) 2 i v f 9.27 m / s ( 9.75 m/ s) Does not depend on the mass of the skier!!

60 W f r f A 2.0-kg block sliding on a rough horizontal surface is attached to one end of a horizontal spring (k = 250 N/m) which has its other end fixed. The block passes through the equilibrium position with a speed of 2.6 m/s and then compresses the spring a distance x from equilibrium Find the distance x that that the spring is compressed. Draw a free body diagram of the forces acting on the spring at the distance x.

61 Loop de loop!

62 System of Objects Change in energy bank due to NC forces!! W U U K K nc m 2 0 m 1

63 Power The time rate of energy transfer: The average power: P W t P de dt The instantaneous power is the limiting value of the average power as Dt approaches zero P lim t 0 W t dw dt dw dr P F F v dt dt

64 Units P Work time J Power has a unit of a Watt: Watt s Energy in terms of Power is Energy Power Time For example, Kilowatt-hour

65 POWER A 2.0-kg block slides down a plane (inclined at 40 with the horizontal) at a constant speed of 5.0 m/s. At what rate is the gravitational force on the block doing work? a. +98 W b. +63 W c. zero d. +75 W e. 75 W

66 Energy Diagrams and Stable Equilibrium: Mass on a Spring F x du dx The x = 0 position is one of stable equilibrium Configurations of stable equilibrium correspond to those for which U(x) is a minimum. x=x max and x=-x max are called the turning points

67 Energy Diagrams and Unstable Equilibrium F x = 0 at x = 0, so the particle is in equilibrium For any other value of x, the particle moves away from the equilibrium position This is an example of unstable equilibrium Configurations of unstable equilibrium correspond to those for which U(x) is a maximum. Ex: A pencil standing on its end. F x du dx

68 Force is Slope! For the potential energy curve shown, (a) determine whether the force F x is positive, negative, or zero at the five points indicated. (b) Indicate points of stable, unstable, and neutral equilibrium. (c) Sketch the curve for F x versus x from x = 0 to x = 9.5 m. F x du dx a) F x is zero at points A, C and E; F x is positive at point B and negative at point D. F x b) A and E are unstable, and C is stable. A B C E x (m) D

69 Varying Gravitational Force Potential Energy & Equipotential F mm G r 1 2 rˆ 2 F r du dr Potential Energy Due to Point Masses: Gradients & Equal Potential Energy Surfaces

70 Varying Electric Force Potential Energy & Equipotential F qq k r 1 2rˆ 2 F r du dr Potential Energy Due to Point Charges: Gradients & Equal Potential Energy Surfaces

71 Gravitational Potential Energy for a variable force As a particle moves from A to B, its gravitational potential energy changes by f U U f Ui W F() r dr Choose the zero for the gravitational potential energy where the force is zero: U i = 0 where r GmM E GmM U f U() r dr 2 r r GM Em Ur () r r r i r Systems bound by Attractive Forces have Negative Energy States! Negative because it is a BOUND system. You must put work into the system to move the mass away from the planet. E i

72 Electric Potential Energy & Potential Difference As a + particle moves from A to B, its electric potential energy changes by B A B E 0 () A U U U W q r dr If the electric field is caused by another point particle then the potential energy difference between A and B is: B Bkq 1 1 U q 0 E() r dr q 0 dr kq 2 0q A A r rb ra In general, 1 1 U kq1q 2( ) r r B A A positive test charge will gain kinetic energy and lose potential energy as it rolls down the electric hill from A to B.

73 V The Electric Force is the Negative of the Gradient of the Electric Potential Energy! q du q ke Fr E ke 2 r dr r The gradient of a scalar field at a point is a vector pointing in the direction of the steepest slope or grade at that point. The steepness of the slope at that point is given by the magnitude of the gradient vector.

74 Electric Potential Energy Plot of a Dipole Charge. Force: How steep the potential energy is!!!

75 A 2.00-kg block situated on a rough incline is connected to a spring of negligible mass having a spring constant of 100 N/m. The pulley is frictionless. The block is released from rest when the spring is unstretched. The block moves 20.0 cm down the incline before coming to rest. Find the coefficient of kinetic friction between block and incline. The picture shows the final state. Take it to be the ground where U=0. h d The gain in internal energy due to friction represents a loss in mechanical energy that must be equal to the change in the kinetic energy plus the change in the potential energy. U U K W g s nc 1 2 mgh kd 0 fd mgd sinq kd mg cosq d kd mgd sinq 2 mg cosq d kd tanq 2mg cosq.115

76 A 20.0-kg block is connected to a 30.0-kg block by a string that passes over a light frictionless pulley. The 30.0-kg block is connected to a spring that has negligible mass and a force constant of 250 N/m. The spring is unstretched when the system is as shown in the figure, and the incline is frictionless. The 20.0-kg block is pulled 20.0 cm down the incline (so that the 30.0-kg block is 40.0 cm above the floor) and released from rest. Find the speed of each block when the 30.0-kg block is 20.0 cm above the floor (that is, when the spring is unstretched.) m 1 m 2 x =.2m Take the equilibrium position shown as the ground for the m2. Take the initial position the ground for m1. The blocks will have the same final speed ( K U ) ( K U U ) ( K U ) ( K U U ) i gi 1 i gi si 2 f fg 1 f gf sf 2 Energy Bank: ( U U ) ( K U ) K gi si 2 f fg 1 2 f :Expenditures m2 gx kx ( m1 m2 ) v m1 gx sin q 2 2 v 2 gx( m m sin q ) kx 2 1 m m v 1.24 m s

77 Energy or Newton s 2 nd Law? A child slides without friction from a height h along a curved water. She is launched from a height h/5 into the pool. Determine her maximum airborne height y in terms of h and.

78 Pendulum Problem A certain pendulum consists of a 1.5-kg mass swinging at the end of a string (length = 2.0 m). At the lowest point in the swing the tension in the string is equal to 20 N. To what maximum height above this lowest point will the mass rise during its oscillation? a. 77 cm b. 50 cm c. 63 cm d. 36 cm e. 95 cm

79 Work Energy with Friction A 1.5-kg block sliding on a rough horizontal surface is attached to one end of a horizontal spring (k = 200 N/m) which has its other end fixed. If this system is displaced 20 cm horizontally from the equilibrium position and released from rest, the block first reaches the equilibrium position with a speed of 2.0 m/s. What is the coefficient of kinetic friction between the block and the horizontal surface on which it slides? a b c d e. 0.17