King Saud University College of Science Physics & Astronomy Dept.

Transcription

1 King Saud University College of Science Physics & Astronomy Dept. PHYS 103 (GENERAL PHYSICS) CHAPTER 7:Energy and Energy Transfer Presented by Nouf Saad Alkathran

2 Imagine a system consisting of a book and the Earth, interacting via the gravitational foce. We do some work on the system by lifting the book slowly from rest through a vertical displacement This work increase the energy of the system and the energy transferred to the system is stored as a potential energy. When the book release or allow to fall, the storage energy (potential energy) transfer to kinetic energy. 2

3 The amount of potential energy in the system is determined by the configuration of the system. Moving members of the system to different positions or rotating them may change the configuration of the system and therefore its potential energy. we can identify the quantity mgy as the gravitational potential energy Ug of the system of an object of mass m and the Earth: 3

4 Gravitational potential energy depends only on the vertical height of the object In solving problems, you must choose a reference configuration for which the gravitational potential energy of the system is set equal to some reference value, which is normally zero. It is often convenient to choose as the reference configuration for zero gravitational potential energy the configuration in which an object is at the surface of the Earth. 4

5 A trophy being shown off by a careless athlete slips from the athlete s hands and drops on his foot. Choosing floor level as the y =0 point of your coordinate system, estimate the change in gravitational potential energy of the trophy Earth system as the trophy falls. Repeat the calculation, using the top of the athlete s head as the origin of coordinates. Suppose the trophy mass m=2 kg, trophy height =1.4 m, his feet height 0.05 m 1) y=0 as a reference 2) Y=2 m as a reference 5

6 Elastic Potential Energy Consider a system consisting of a block and a spring The force that the spring exerts on the block is given by Fs = -kx The external work done by an applied force Fapp on the block spring system is given by The elastic potential energy is defined by 6

7 The elastic potential energy of the system can be thought of as the energy stored in the deformed spring. The elastic potential energy stored in a spring is zero whenever the spring is undeformed (x = 0). Because the elastic potential energy is proportional to x2, we see that Us is always positive in a deformed spring. 7

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10 A ball is connected to a light spring suspended vertically. When pulled downward from its equilibrium position and released, the ball oscillates up and down. (i) In the system of the ball, the spring, and the Earth, what forms of energy are there during the motion? (ii) In the system of the ball and the spring, what forms of energy are there during the motion? (a) kinetic and elastic potential (b) kinetic and gravitational potential (c) kinetic, elastic potential, and gravitational potential (d) elastic potential and gravitational potential. 10

11 Internal energy: The energy associated with the temperature of a system The force is to the right and the displacement is to the right. So, the work done is positive. Then the system has kinetic energy The internal energy of the system increases the book and the surface are warmer than before. When the book stops, the kinetic energy has been completely transformed to internal energy 11

12 The amount of internal energy in the system after the book has stopped is equal to the amount of kinetic energy in the system at the initial instant. This equality is described by an important principle called conservation of energy. The work done by gravitational force and friction force: The path makes no difference when we consider the work done by the gravitational force, but it does make a difference when we consider the energy transformation due to friction forces. We can use this varying dependence on path to classify forces as either conservative or nonconservative.of the two forces just mentioned, the gravitational force is conservative and the friction force is nonconservative. 12

13 Conservative Forces Conservative forces have these two equivalent properties: 1. The work done by a conservative force on a particle moving between any two points is independent of the path taken by the particle. 2. The work done by a conservative force on a particle moving through any closed path is zero. 13