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Physics 151: Lecture 16 Today’s Agenda

Physics 151: Lecture 16 Today’s Agenda. Today’s Topics: Conservation of mechanical energy Nonconservative forces and loss of energy from a system Other relationships for potential energy. E = K + U is constant !!!. See text: 8-4. Conservation of Energy.

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Physics 151: Lecture 16 Today’s Agenda

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  1. Physics 151: Lecture 16Today’s Agenda • Today’s Topics: • Conservation of mechanical energy • Nonconservative forces and loss of energy from a system • Other relationships for potential energy

  2. E = K + U is constant!!! See text: 8-4 Conservation of Energy • If only conservative forces are present, the total energy (sum of potential and kinetic energies) of a systemis conserved. E = K + U E = K + U = W + U using K = W = W + (-W) = 0 using U = -W Both K and U can change, but E = K + U remainsconstant.

  3. See text: Example 8.3 Example: The simple pendulum. • Suppose we release a mass m from rest a distance h1 above its lowest possible point. • What is the maximum speed of the mass and wheredoes this happen ? • To what height h2 does it rise on the other side ? m h1 h2 v

  4. See text: Example 8.3 Example: The simple pendulum. • Energy is conserved since gravity is a conservative force (E = K + U is constant) • Choose y = 0 at the bottom of the swing, and U = 0 at y = 0 (arbitrary choice).E = 1/2mv2 + mgy. y h1 h2 y=0 v

  5. Car has mass m h ? R Lecture 16, ACT 1The Roller Coaster • I have built a Roller Coaster . A motor tugs the cars to the top and then they are let go and are in the hands of gravity. To make the following loop, how high do I have to let the release the car ?? A) 2R B) 5R C) 5/2 R D) none of the above

  6. Another Problem Involving a Spring A large spring is used to stop the cars after they come down the last hill of a roller coaster. The cars start at rest at the top of the hill and are caught by a mechanism at the instant their velocities at the bottom are zero. Compare the compression of the spring, xA, for a fully loaded car with that, xB, for a lightly loaded car when mA= 2mB. a. xA = 1/2 xB. b. xA = xB. c. xA = (2)1/2 xB. d. xA = 2 xB. e. xA = 4 xB.

  7. See text: 8.5 Non-conservative Forces: • If the work done does not depend on the path taken, the force involved is said to be conservative. • If the work done does depend on the path taken, the force involved is said to be non-conservative. • An example of a non-conservative force is friction: • Pushing a box across the floor, the amount of work that is done by friction depends on the path taken. • Work done is proportional to the length of the path !

  8. Generalized Work Energy Theorem: WNC = K + U = E

  9. Lecture 16, ACT 2Stones and Friction • I throw a stone into the air. While in flight it feels the force of gravity and the frictional force of the air resistance. The time the stone takes to reach the top of its flight path (i.e. go up) is, A) larger than B) equal to C) less than The time it takes to return from the top (i.e. go down).

  10. Lecture 16, ACT 2Solution • We know that WNC = K + U = E • Thus as time progresses the amount of energy in the earth-rock system is continually decreasing • If I consider the way up versus the way down at a given height, then Eup = Kup + Uup > Edown = Kdown + Udown • But, at a given height U is always the same. So, Kup > Kdown • v at a given height is always less on the way down The answer is (C) it takes less time to go up

  11. Same problem but with numbers: • A 2.0-kg mass is projected vertically upward from ground level with an initial speed of 30 m/s. The mass rises to a maximum height of 35 m above ground level. How much work is done on the mass by air resistance between the point of projection and the point of maximum height? a. –0.21 kJ b. +0.21 kJ c. –0.40 kJ d. +0.69 kJ e. –0.69 kJ

  12. Question • As an object moves from point A to point B only two forces act on it: one force is nonconservative and does –30 J of work, the other force is conservative and does +50 J of work. Between A and B, • the kinetic energy of object increases, mechanical energy decreases. • the kinetic energy of object decreases, mechanical energy decreases. • the kinetic energy of object decreases, mechanical energy increases. • the kinetic energy of object increases, mechanical energy increases. • None of the above.

  13. Question - 2 • As an object moves from point A to point B only two forces act on it: one force is conservative and does –70 J of work, the other force is nonconservative and does +50 J of work. Between A and B, • the kinetic energy of object increases, mechanical energy increases. • the kinetic energy of object decreases, mechanical energy increases. • the kinetic energy of object decreases, mechanical energy decreases. • the kinetic energy of object increases, mechanical energy decreases. • None of the above.

  14. ACT- 2 • Objects A and B, of mass M and 2M respectively, are each pushed a distance d straight up an inclined plane by a force F parallel to the plane. The coefficient of kinetic friction between each mass and the plane has the same value . At the highest point, • K A > K B . • K A = K B . • K A < K B . • The work done by F on A is greater than the work done on B. • The work done by F on A is less than the work done on B.

  15. . . . . Lecture 16, ExampleSkateboard • Let’s now suppose that the surface is not frictionless and the same skateboarder reach the speed of 7.0 m/s at bottom of the hill. What was the work done by friction on the skateboarder ? Conservation of Total Energy : K1 + U1 = K2 + U2 Wf + m = 25 kg Wf + 0 + mgR = 1/2mv2 + 0 Wf = 1/2mv 2 - mgR R=3 m Wf = (1/2 x25 kg x (7.0 m/s2)2 - - 25 kg x 10m/s2 3 m) Wf = 613 - 735 J = - 122 J Total mechanical energy decreased by 122 J !

  16. See Text: section 8.6 Conservative Forces and Potential Energy • We have defined potential energy for conservative forces DU = -W • But we also now that W = FxDx • Combining these two, DU = -FxDx • Letting small quantities go to infinitesimals, dU = -Fxdx • Or, Fx = -dU/dx

  17. See Text: section 8.6 Examples of the U - F relationship • Remember the spring, • U = (1/2)kx2 • Do the derivative • Fx = - d ( (1/2)kx2) / dx • Fx = - 2 (1/2) kx • Fx = -kx

  18. See Text: section 8.6 Examples of the U - F relationship • Remember gravity, • Fy = mg • Do the integral • U = -F dy • U = - (-mg) dy • U = mgy + C • DU = U2 – U1 = (mgy2 + C) – (mgy1 + C) = mgDy

  19. Recap of today’s lecture • Conservation of Mechanical Energy • Non-conservative forces and loss of energy from a system • Other relationships for potential energy

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