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Energy

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**Energy can change from one form to another without a net loss or gain.**

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Energy may be the most familiar concept in science, yet it is one of the most difficult to define. We observe the effects of energy when something is happening—only when energy is being transferred from one place to another or transformed from one form to another.

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**So far we have done: velocity, acceleration, force, and blaablaablaa. **

All of the above have the same meaning as they do in everyday human life. Now we will do a wacky one……. Work

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Work is done when a net force acts on an object and the object moves in the direction of the net force.

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Work is equal to the Force applied to an object and the distance the object moves. The force and distance must be parallel to each other. The Joule is the unit of work. 1 Joule = 1 Newton * 1 meter 1J = 1 N * m

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9.1 Work If we lift two loads, we do twice as much work as lifting one load the same distance, because the force needed is twice as great. If we lift one load twice as far, we do twice as much work because the distance is twice as great.

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**Work is done in lifting the barbell**

Work is done in lifting the barbell. If the barbell could be lifted twice as high, the weight lifter would have to do twice as much work.

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9.1 Work While the weight lifter is holding a barbell over his head, he may get really tired, but he does no work on the barbell. Work may be done on the muscles by stretching and squeezing them, but this work is not done on the barbell. When the weight lifter raises the barbell, he is doing work on it.

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9.1 Work Some work is done to change the speed of an object. Bringing an automobile up to speed or in slowing it down involves work. In both categories, work involves a transfer of energy between something and its surroundings.

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9.1 Work think! Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?

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A 10-N forces is applied to push a block across a friction free surface for a displacement of 5.0 m to the right.

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A 2-kg object is sliding at constant speed across a friction free surface for a displacement of 5 m to the right.

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**A force of 50 N acts on the block at the angle shown in the diagram**

A force of 50 N acts on the block at the angle shown in the diagram. The block moves a horizontal distance of 3.0 m.

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Energy and Work A body experiences a change in energy when one or more forces do work on it. A force does positive work on a body when the force and the displacement are at least partially aligned. Maximum positive work is done when a force and a displacement are in exactly the same direction. If a force causes no displacement, it does no work. Normal force, centripetal forces Forces can do negative work if they are pointed opposite the direction of the displacement.

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**Work and a Pulley System**

A pulley system, which has at least one pulley attached to the load, can be used to reduce the force necessary to lift a load. Amount of work done in lifting the load is not changed. The distance the force is applied over is increased, thus the force is reduced, since W = Fd. F m

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Net Work or Total Work An object can be subject to many forces at the same time, and if the object is moving, the work done by each force can be individually determined. At the same time one force does positive work on the object, another force may be doing negative work, and yet another force may be doing no work at all. The net work, or total, work done on the object (Wnet or Wtot) is the scalar sum of the work done on an object by all forces acting upon the object. Wnet = ΣWi

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Kinetic Energy Kinetic energy is the energy of motion. An object which has motion - whether it be vertical or horizontal motion - has kinetic energy.

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Kinetic Energy Kinetic energy is one form of mechanical energy, which is energy we can easily see and characterize. Kinetic energy is due to the motion of an object. K = ½ m v2 K: Kinetic Energy in Joules. m: mass in kg v: speed in m/s In vector form, K = ½ m v•v

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9.5 Kinetic Energy When you throw a ball, you do work on it to give it speed as it leaves your hand. The moving ball can then hit something and push it, doing work on what it hits.

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Sample Problem A net force of 320 N acts over 1.3 m on a 0.4 kg particle moving at 2.0 m/s. What is the speed of this particle after this interaction?

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**The Work-Energy Theorem**

Wnet = ΔK When net work due to all forces acting upon an object is positive, the kinetic energy of the object will increase. When net work due to all forces acting upon an object is negative, the kinetic energy of the object will decrease. When there is no net work acting upon an object, the kinetic energy of the object will be unchanged. (Note this says nothing about the kinetic energy.)

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Potential energy A type of mechanical energy possessed by an object by virtue of its position or configuration. Represented by the letter U. Examples: Gravitational potential energy, Ug = mgh Spring potential energy , Us = ½ kx2

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9.4 Potential Energy The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case. The boulder is lifted with 100 N of force. The boulder is pushed up the 4-m incline with 50 N of force. The boulder is lifted with 100 N of force up each 0.5-m stair.

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**Gravitational Potential Energy (Ug)**

The change in gravitational potential energy is the negative of the work done by gravitational force on an object when it is moved. For objects near the earth’s surface, the gravitational pull of the earth is roughly constant, so the force necessary to lift an object at constant velocity is equal to the weight, so we can say ΔUg = Wg = mgh Note that this means we have defined the point at which Ug = 0, which we can do arbitrarily in any given problem close to the earth’s surface. h Fapp mg

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9.2 Power Power equals the amount of work done divided by the time interval during which the work is done.

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9.2 Power When carrying a load up some stairs, you do the same amount of work whether you walk or run up the stairs. Power is the rate at which work is done.

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9.2 Power A high-power engine does work rapidly. An engine that delivers twice the power of another engine does not necessarily produce twice as much work or go twice as fast. Twice the power means the engine can do twice the work in the same amount of time or the same amount of work in half the time. A powerful engine can get an automobile up to a given speed in less time than a less powerful engine can.

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9.2 Power The unit of power is the joule per second, also known as the watt. One watt (W) of power is expended when one joule of work is done in one second. One kilowatt (kW) equals 1000 watts. One megawatt (MW) equals one million watts.

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9.2 Power The three main engines of the space shuttle can develop 33,000 MW of power when fuel is burned at the enormous rate of 3400 kg/s.

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Sample Problem A 1000-kg space probe lifts straight upward off the planet Zombie, which is without an atmosphere, at a constant speed of 3.0 m/s. What is the power expended by the probe’s engines? The acceleration due to gravity of Zombie is ½ that of earth’s.

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Sample Problem Develop an expression for the power output of an airplane cruising at constant speed v in level flight. Assume that the aerodynamic drag force is given by FD = bv2. By what factor must the power be increased to increase airspeed by 25%?

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**9.7 Conservation of Energy**

Part of the PE of the wound spring changes into KE. The remaining PE goes into heating the machinery and the surroundings due to friction. No energy is lost.

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**9.7 Conservation of Energy**

Everywhere along the path of the pendulum bob, the sum of PE and KE is the same. Because of the work done against friction, this energy will eventually be transformed into heat.

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**9.7 Conservation of Energy**

When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground.

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**Law of Conservation of Energy**

The system is isolated and boundary allows no exchange with the environment. E = U + K + Eint = Constant No mass can enter or leave! No energy can enter or leave! Energy is constant, or conserved!

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**More about force types Conservative forces: Non-conservative forces:**

Work in moving an object is path independent. Work in moving an object along a closed path is zero. Work is directly related to the change in potential energy Ex: gravity, springs Non-conservative forces: Work is path dependent. Work along a closed path is NOT zero. Work may be related to a change in mechanical energy, or thermal energy Ex: friction, drag

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Dr. Steve Peterson Steve.peterson@uct.ac.za Physics 1025F Mechanics ENERGY Dr. Steve Peterson Steve.peterson@uct.ac.za.

Dr. Steve Peterson Steve.peterson@uct.ac.za Physics 1025F Mechanics ENERGY Dr. Steve Peterson Steve.peterson@uct.ac.za.

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