Chapter 8 Conservation of Energy 1 Energy Review

Chapter 8 Conservation of Energy 1

Energy Review n Kinetic Energy n n Potential Energy n n n Associated with movement of members of a system Determined by the configuration of the system Gravitational and Elastic Internal Energy n Related to the temperature of the system 2

Types of Systems n Nonisolated systems n n n Energy can cross the system boundary in a variety of ways Total energy of the system changes Isolated systems n n Energy does not cross the boundary of the system Total energy of the system is constant 3

Ways to Transfer Energy Into or Out of A System n n n 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 4

More Ways to Transfer Energy Into or Out of A System n n n 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 5

Examples of Ways to Transfer Energy Section 8. 1 6

Conservation of Energy n Energy is conserved n n This means that energy cannot be created nor destroyed If the total amount of energy in a system changes, it can only be due to the fact that energy has crossed the boundary of the system by some method of energy transfer 7

Conservation of Energy, cont. n Mathematically, n n Esystem is the total energy of the system T is the energy transferred across the system boundary n n n Established symbols: Twork = W and Theat = Q Others just use subscripts The Work-Kinetic Energy theorem is a special case of Conservation of Energy n The full expansion of the above equation gives 8

Isolated System n For an isolated system, DEmech = 0 n n Remember Emech = K + U This is conservation of energy for an isolated system with no nonconservative forces acting If nonconservative forces are acting, some energy is transformed into internal energy Conservation of Energy becomes DEsystem = 0 n n Esystem is all kinetic, potential, and internal energies This is the most general statement of the isolated system model 9

Conservation of Mechanical Energy, example n n Look at the work done by the book as it falls from some height to a lower height Won book = DKbook Also, W = mgyi – mgyf So, DK = mgyi – mgyf =-(mgyf – mgyi)=-(Uf-Ui) =-DUg 10

Isolated System, cont n The changes in energy can be written out and rearranged n n Remember, this applies only to a system in which conservative forces act 11

Problem Solving Strategy – Conservation of Mechanical Energy for an Isolated System n Conceptualize n n n Form a mental representation Imagine what types of energy are changing in the system Categorize n n Define the system It may consist of more than one object and may or may not include springs or other sources of storing potential energy 12

Problem Solving Strategy, cont n Categorize, cont n Determine if any energy transfers occur across the boundary of your system n n n If there are transfers, use DEsystem = ST If there are no transfers, use DEsystem = 0 Determine if there any nonconservative forces acting n If not, use the principle of conservation of mechanical energy 13

Problem-Solving Strategy, 2 n Analyze n n Choose configurations to represent initial and final configuration of the system For each object that changes height, identify the zero configuration for gravitational potential energy For each object on a spring, the zero configuration for elastic potential energy is when the object is in equilibrium If more than one conservative force is acting within the system, write an expression for the potential energy associated with each force 14

Problem-Solving Strategy, 3 n Analyze, cont n n n Write expressions for total initial mechanical energy and total final mechanical energy Set them equal to each other Finalize n n Make sure your results are consistent with your mental representation Make sure the values are reasonable and consistent with everyday experience 15

Example – Free Fall n n Determine the speed of the ball at y above the ground Conceptualize n n Use energy instead of motion Categorize n n System is isolated Only force is gravitational which is conservative 16

Example – Free Fall, cont n n Analyze n Apply Conservation of Energy n Kf + Ugf = Ki + Ugi n Ki = 0, the ball is dropped n Solving for vf Finalize n The equation for vf is consistent with the results obtained from kinematics 17

A Grand Entrance 18

Spring Loaded Gun n Conceptualize n n The projectile starts from rest Speeds up as the spring pushes against it As it leaves the gun, gravity slows it down Categorize n n System is projectile, gun, and Earth Model as a system with no nonconservative forces acting 19

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Example – Spring Gun, cont n Analyze n n n Projectile starts from rest, so Ki = 0 Choose zero for gravitational potential energy where projectile leaves the gun Elastic potential energy will also be 0 here After the gun is fired, the projectile rises to a maximum height, where its kinetic energy is 0 Finalize n n Did the answer make sense Note the inclusion of two types of potential energy 21

Kinetic Friction n Kinetic friction can be modeled as the interaction between identical teeth The frictional force is spread out over the entire contact surface The displacement of the point of application of the frictional force is not calculable 22

Work – Kinetic Energy Theorem n n n It is valid for a particle or an object that can be modeled as a particle When a friction force acts, you cannot easily calculate the work done by friction However, Newton’s Second Law is still valid 23

Work – Kinetic Energy With Friction n In general, if friction is acting in a system: n n This is a modified form of the work – kinetic energy theorem n n Use this form when friction acts on an object If friction is zero, this equation becomes the same as Conservation of Mechanical Energy 24

Including Friction, final n n A friction force transforms kinetic energy in a system to internal energy The increase in internal energy of the system is equal to its decrease in kinetic energy n 25

Example – Block on Rough Surface n n The block is pulled by a constant force over a rough horizontal surface Conceptualize n n The rough surface applies a friction force on the block The friction force is in the direction opposite to the applied force 26

Example – Rough Surface cont n Categorize n n Model the block-surface system as nonisolated with a nonconservative force acting Analyze n n Neither the normal nor gravitational forces do work on the system Vertical direction – apply particle in equilibrium model n n Find the magnitude of the friction force Solve for final speed 27

Example – Rough Surface, final n Finalize n n Less than value found in example without friction What if? n n n Suppose the force is applied at an angle q = 0 gives greatest speed if no friction At an non-zero angle, the frictional force is also reduced 28

Example – Block-spring System n The problem n n n Conceptualize n n The mass is attached to a spring, the spring is compressed and then the mass is released A constant friction force acts The block will be pushed by the spring and move off with some speed Categorize n n Block and surface is the system System is nonisolated 29

Example – Spring-block, cont n Analyze n n n Evaluate ƒk d Evaluate SWother forces Finalize n Think about the result 30

Adding Changes in Potential Energy n If friction acts within an isolated system n n DU is the change in all forms of potential energy If friction acts within a nonisolated system 31

Problem Solving Strategy with Nonconservative Forces n Conceptualize n n Form a mental representation of what is happening Categorize n Define the system n n Determine if any nonconservative forces are present n n May consist of more than one object If not, use principle of conservation of mechanical energy Determine if any work is done across the boundary of your system by forces other than friction 32

Problem Solving, cont n Analyze n n Identify the initial and final conditions of the system Identify the configuration for zero potential energy n n n Include gravitational potential energy and spring elastic potential energy points If there is more than one conservative force, write an expression for the potential energy associated with each force Finalize n Make sure your results are consistent with your mental representation 33

Example – Ramp with Friction n Problem: the crate slides down the rough ramp n n Conceptualize n n Find speed at bottom Energy considerations Categorize n n Identify the crate, the surface, and the Earth as the system Isolated system with nonconservative force acting 34

Example – Ramp, cont n Analyze n n n Let the bottom of the ramp be y = 0 At the top: Ei = Ki + Ugi = 0 + mgyi At the bottom: Ef = Kf + Ugf = ½ m vf 2 + mgyf Then DEmech = Ef – Ei = -ƒk d Solve for vf Finalize n n Could compare with result if ramp was frictionless The internal energy of the system increased 35

Example – Spring Mass Collision n n Without friction, the energy continues to be transformed between kinetic and elastic potential energies and the total energy remains the same If friction is present, the energy decreases n DEmech = -ƒkd 36

Example – Spring Mass, 2 n Conceptualize n n Categorize n n n All motion takes place on a horizontal plane n So no changes in gravitational potential energy The system is the block, the surface, and the system The system is isolated but now involves a nonconservative force Analyze n Before the collision, the total energy is kinetic 37

Problem – Spring Mass 3 n Analyze n n Before the collision, the total energy is kinetic When the spring is totally compressed, the kinetic energy is zero and all the energy is elastic potential Total mechanical energy is conserved Finalize n Decide which root has physical meeting 38

Problem – Spring Mass 4 n Now add friction n Categorize n n Analyze n n Now isolated with a nonconservative force Use DEmech = -ƒk d Finalize n The value is less than the case for no friction n As expected 39

Example – Connected Blocks n Conceptualize n n Configurations of the system when at rest are good candidates for initial and final points Categorize n n The system consists of the two blocks, the spring, and Earth System is isolated with a nonconservative force acting 40

Example – Blocks, cont n Categorize, cont n n Gravitational and potential energies are involved The kinetic energy is zero if our initial and final configurations are at rest Model the sliding block as a particle in equilibrium in the vertical direction Analyze n Two forms of potential energy are involved 41

Connected Blocks, cont n Analyze, cont n n Block 2 undergoes a change in gravitational potential energy The spring undergoes a change in elastic potential energy The coefficient of kinetic friction can be measured Finalize n This allows a method for measuring the coefficient of kinetic friction 42

Instantaneous Power n n n Power is the time rate of energy transfer The instantaneous power is defined as Using work as the energy transfer method, this can also be written as 43

Power n n The time rate of energy transfer is called power The average power is given by n when the method of energy transfer is work 44

Instantaneous Power and Average Power n n The instantaneous power is the limiting value of the average power as Dt approaches zero The power is valid for any means of energy transfer 45

Units of Power n The SI unit of power is called the watt n n A unit of power in the US Customary system is horsepower n n 1 hp = 746 W Units of power can also be used to express units of work or energy n 1 k. Wh = (1000 W)(3600 s) = 3. 6 x 106 J 46

Power delivered by an elevator motor 47