Potential energy and conservation of energy

Transcription

1 Chapter 8 Potential energy and conservation of energy Copyright

2 8.1_2 Potential Energy and Work Potential energy U is energy that can be associated with the configuration (arrangement) of a system of objects that exert forces on one another. Physics determines how potential energy is calculated, to account for stored energy. Gravitational potential energy Some forms of potential energy: Elastic The change DU in potential energy potential energy (gravitational, elastic, etc) is defined as being equal to the negative of the work done on the object by the force (gravitational, elastic, etc)

3 8.2 Work and Potential Energy: Conservative and non-conservative forces Suppose: 1.A system consists of two or more objects. 2. A force acts between a particle-like object in the system and the rest of the system. 3. When the system configuration changes, the force does work (call it W 1 ) on the object, transferring energy between the kinetic energy of the object, K, and some other type of energy of the system. 4. When the configuration change is reversed, the force reverses the energy transfer, doing work W 2 in the process. In a situation in which W 1 =-W 2 is always true, the other type of energy is a potential energy and the force is said to be a conservative force. o Examples: gravitational force, spring force. o Otherwise we could not speak of their potential energies.

4 8.2 Work and Potential Energy: Conservative and non-conservative forces A force that is not conservative is called a non-conservative force. o Examples: kinetic friction force, drag force. o o o Kinetic energy of a moving particle is transferred to heat by friction. Thermal energy cannot be recovered back into kinetic energy of the object via the friction force.. Therefore the force is not conservative, thermal energy is not a potential energy. When only conservative forces act on a particle, we find many problems can be simplified:

5 8.3 Path Independence of Conservative Forces The net work done by a conservative force on a particle moving around any closed path is zero. If the work done from a to b along path 1 as W ab,1 and the work done from b back to a along path 2 as W ba,2. If the force is conservative, then the net work done during the round trip must be zero If the force is conservative,

6 8.3 Path Independence of Conservative Forces Sample problem: Equivalent paths f or calculating work, slippery cheese Calculations: Let us choose the dashed path in Fig. 8-5b; it consists of two straight segments. Along the horizontal segment, the angle f is a constant 90. Even though we do not know the displacement along that horizontal segment, the work W h done there is Along the vertical segment, the displacement d is 0.80 m and, with and both downward, the angle is a constant = 0. Thus, for the work W v done along the vertical part of the dashed path, The total work done on the cheese by F g as the cheese moves from point a to point b along the dashed path is then This is also the work done as the cheese slides along the track from a to b.

7 8.4 Determining Potential Energy Values For the most general case, in which the force may vary with position, we may write the work W :

8 8.4 Determining Potential Energy Values: Gravitational Potential Energy A particle with mass m moving vertically along a y axis (the positive direction is upward). The corresponding change in the gravitational potential energy of the particle-earth system is: Dy m y y f y i for The gravitational potential energy associated with a particle-earth system depends only on the vertical position y (or height) of the particle relative to the reference position y =0, not on the horizontal position.

9 8.4 Determining Potential Energy Values: Elastic Potential Energy In a block spring system, the block is moving on the end of a spring of spring constant k. The corresponding change in the elastic potential energy of the block-spring system is If the reference configuration is when the spring is at its relaxed length, and the block is at x i = 0.

10 8.4 Determining Potential Energy Values Sample problem: Gravitational Potential Energy

11 8.5 Conservation of Mechanical Energy Principle of conservation of energy: In an isolated system where only conservative forces cause energy changes, the kinetic energy and potential energy can change, but their sum, the mechanical energy E mec of the system, cannot change.

12 8.5 Conservation of Mechanical Energy This is the principle of the conservation of mechanical energy: This is very powerful tool: One application: o Choose the lowest point in the system as U = 0 o Then at the highest point U = max, and K = min

13 8.5 Conservation of Mechanical Energy A pendulum swings back and forth. Potential and kinetic energies of the pendulum-earth system vary, but the mechanical energy E mec of the system remains constant. The energy E mec can be described as continuously shifting between the kinetic and potential forms. If the swinging involved a frictional force then E mec would not be conserved, and eventually the pendulum would stop.

14 8.5 Conservation of Mechanical Energy Sample problem: Water Slide

15 8.7 Work done on a System by an External Force We can extend work on an object to work on a system. Work is energy transferred to or from a system by means of an external force acting on that system. For a system of more than 1 particle, work can change both K and U, or other forms of energy of the system. frictionless system system with friction

16 8.7 Work done on a System by an External Force FRICTION NOT INVOLVED FRICTION INVOLVED

17 8.7 Work done on a System by an External Force Sample problem: Change in Thermal Energy

18 8.8 Conservation of Energy: Law of Conservation of Energy Energy transferred between systems can always be accounted for. The law of conservation of energy concerns o The total energy E of a system o Which includes mechanical, thermal, and other internal energy The total energy E of a system can change only by amounts of energy that are transferred to or from the system. Considering only energy transfer through work: where: ΔE mec is any change in the mechanical energy of the system, ΔE th is any change in the thermal energy of the system, ΔE int is any change in any other type of internal energy of the system.

19 8.8 Conservation of Energy An isolated system is one for which there can be no external energy transfer. The total energy E of an isolated system cannot change. Energy transfers may happen internal to the system. Or, for two instants of time:

20 8.8 Conservation of Energy: External Forces and Internal Energy Transfers External forces can act on a system without doing work. The skater pushes herself away from the wall She turns internal chemical energy in her muscles into kinetic energy An external force can change the kinetic energy or potential energy of an object without doing work on the object, i.e. without transferring energy to the object. Instead, the force causes transfers of energy from one type to another.

21 8.8 Conservation of Energy: Power We can expand the definition of power In general, power is the rate at which energy is transferred by a force from one type to another If energy ΔE is transferred in time Δt, the average power is: And the instantaneous power is:

22 8.8 Conservation of Energy Sample problem: Energy, Friction, Spring and Tamales System: The package-spring-floor-wall system includes all these forces and energy transfers in one isolated system. From conservation of energy, Forces: The normal force on the package from the floor does no work on the package. For the same reason, the gravitational force on the package does no work. As the spring is compressed, a spring force does work on the package. The spring force also pushes against a rigid wall. There is friction between the package and the floor, and the sliding of the package across the floor increases their thermal energies.

23 Example:

24 8 Summary Conservative Forces Net work on a particle over a closed path is 0 Potential Energy Energy associated with the configuration of a system and a conservative force Eq. (8-6) Gravitational Potential Energy Energy associated with Earth + a nearby particle Eq. (8-9) Elastic Potential Energy Energy associated with compression or extension of a spring Eq. (8-11)

25 8 Summary Mechanical Energy Potential Energy Curves Eq. (8-12) For only conservative forces within an isolated system, mechanical energy is conserved Work Done on a System by an External Force Without/with friction: Eq. (8-26) At turning points a particle reverses direction Eq. (8-22) At equilibrium, slope of U(x) is 0 Conservation of Energy The total energy can change only by amounts transferred in or out of the system Eq. (8-33) Eq. (8-35)

26 8 Summary Power The rate at which a force transfers energy Average power: Eq. (8-40) Instantaneous power: Eq. (8-41)

27 8 Solved Problems 1. A 5.0 g marble is fired vertically upward using a spring gun. The spring must be compressed 8.0 cm if the marble is to just reach a target 20 m above the marble's position on the compressed spring. (a) What is the change ΔU g in the gravitational potential energy of the marble-earth system during the 20 m ascent? (b) What is the change in the elastic potential energy of the spring during its launch of the marble? (c) What is the spring constant of the spring?

28 8 Solved Problems 2. In Figure, a runaway truck with failed brakes is moving downgrade at 130 km/h just before the driver steers the truck up a frictionless emergency escape ramp with an inclination of θ=15 o. The truck's mass is 1.2x10 4 kg. (a) What minimum length L must the ramp have if the truck is to stop (momentarily) along it? (Assume the truck is a particle, and justify that assumption.) Does the minimum length L increase, decrease, or remain the same if (b) the truck's mass is decreased and (c) its speed is decreased?

29 8 Solved Problems

30 8 Solved Problems 3.Figure shows an 8.00 kg stone at rest on a spring. The spring is compressed 10.0 cm by the stone. (a) What is the spring constant? (b) The stone is pushed down an additional 30.0 cm and released. What is the elastic potential energy of the compressed spring just before that release? (c) What is the change in the gravitational potential energy of the stone-earth system when the stone moves from the release point to its maximum height? (d) What is that maximum height, measured from the release point?

31 8 Solved Problems 4. The potential energy of a diatomic molecule (a two atom system like H 2 or O 2 ) is given by where r is the separation of the two atoms of the molecule and and A are p and B are positive constants. This potential energy is associated with the force that binds the two atoms together. (a) Find the equilibrium separation-that is, the distance between the atoms at which the force on each atom is zero. Is the force repulsive (the atoms are pushed apart) or attractive (they are pulled together) if their separation is (b) smaller and (c) larger than the equilibrium separation?

32 8 Solved Problems 5. A horizontal force of magnitude 35.0 N pushes a block of mass 4.00 kg across a floor where the coefficient of kinetic friction is (a) How much work is done by that applied force on the block-floor system when the block slides through a displacement of 3.00 m across the floor? (b) During that displacement, the thermal energy of the block increases by 40.0 J. What is the increase in thermal energy of the floor? (c) What is the increase in the kinetic energy of the block?

33 8 Solved Problems 6.In Figure, a block slides down an incline. As it moves from point A to point B, which are 5.0 m apart, force F acts on the block, with magnitude 2.0 N and directed down the incline. The magnitude of the frictional force acting on the block is 10 N. If the kinetic energy of the block increases by 35 J between A and B, how much work is done on the block by the gravitational force as the block moves from A to B?

34 8 Solved Problems 7.In Figure, a 3.5 kg block is accelerated from rest by a compressed spring of spring constant 640 N/m. The block leaves the spring at the spring's relaxed length and then travels over a horizontal floor with a coefficient of kinetic friction μ k. The frictional force stops the block in distance D=7.8 m.what are (a) the increase in the thermal energy of the block-floor system, (b) the maximum kinetic energy of the block, and (c) the original compression distance of the spring?

35 1-8 Scientific Notation Additional Materials

36 8.6 Reading a Potential Energy Curve A plot of U(x), the potential energy function of a system containing a particle confined to move along an x axis. There is no friction, so mechanical energy is conserved. A plot of the force F(x) acting on the particle, derived from the potential energy plot by taking its slope at various points.

37 8.6 Reading a Potential Energy Curve To find K(x) at any place, take the total mechanical energy (constant) and subtract U(x) Places where K = 0 are turning points o There, the particle changes direction (K cannot be negative) At equilibrium points, the slope of U(x) is 0 A particle in neutral equilibrium is stationary, with potential energy only, and net force = 0 o o If displaced to one side slightly, it would remain in its new position Example: a marble on a flat tabletop

38 8.6 Reading a Potential Energy Curve A particle in unstable equilibrium is stationary, with potential energy only, and net force = 0 o o If displaced slightly to one direction, it will feel a force propelling it in that direction Example: a marble balanced on a bowling ball A particle in stable equilibrium is stationary, with potential energy only, and net force = 0 o o If displaced to one side slightly, it will feel a force returning it to its original position Example: a marble placed at the bottom of a bowl

39 8.6 Reading a Potential Energy Curve Plot (a) shows the potential U(x) Plot (b) shows the force F(x) If we draw a horizontal line, (c) or (f) for example, we can see the range of possible positions x < x 1 is forbidden for the E mec in (c): the particle does not have the energy to reach those points

40 8.6 Reading a Potential Energy Curve: Equilibrium Points The U(x) plot with three possible values of E mec Turning point Turning point Turning point E mec = 4 J E mec = 3 J E mec =1J Turning point U = 4 J K = 0 Turning point Neutral equilibrium Unstable equilibrium U = 3 J K = 0 U = 1 J K = 0 (can not move) Stable equilibrium U = 0 K = 1 J (can move but only between the turning points)

41 8.6 Reading a Potential Energy Curve Sample problem: Reading a Potential Energy Graph

42 8-1_4 Potential Energy and Work Learning Objectives 8.01 Distinguish a conservative force force from a nonconservative force For a particle moving between two points, identify that the work done by a conservative force does not depend on which path the particle takes Calculate the gravitational potential energy of a particle (or, more properly, a particle- Earth system) Calculate the elastic potential energy of a block-spring system.

43 8-5 Conservation of Mechanical Energy Learning Objectives 8.05 After first clearly defining which objects form a system, identify that the mechanical energy of the system is the sum of the kinetic energies and potential energies of those objects For an isolated system in which only conservative forces act, apply the conservation of mechanical energy to relate the initial potential and kinetic energies to the potential and kinetic energies at a later instant.

44 8-6 Reading a Potential Energy Curve Learning Objectives 8.07 Given a particle's potential energy as a function of position x, determine the force on the particle Given a graph of potential energy versus x, determine the force on a particle On a graph of potential energy versus x, superimpose a line for a particle's mechanical energy and determine kinetic energy for any given value of x If a particle moves along an x axis, use a potential-energy graph for that axis and the conservation of mechanical energy to relate the energy values at one position to those at another position On a potential-energy graph, identify any turning points and any regions where the particle is not allowed because of energy requirements Explain neutral equilibrium, stable equilibrium, and unstable equilibrium.

45 8-7 Work Done on a System by an External Force Learning Objectives 8.13 When work is done on a system by an external force with no friction involved, determine the changes in kinetic energy and potential energy When work is done on a system by an external force with friction involved, relate that work to the changes in kinetic energy, potential energy, and thermal energy.

46 8-8 Conservation of Energy Learning Objectives 8.15 For an isolated system (no net external force), apply the conservation of energy to relate the initial total energy (energies of all kinds) to the total energy at a later instant For a nonisolated system, relate the work done on the system by a net external force to the changes in the various types of energies within the system Apply the relationship between average power, the associated energy transfer, and the time interval in which that transfer is made Given an energy transfer as a function of time (either as an equation or graph), determine the instantaneous power (the transfer at any given instant).