holt physics chapter 5 outline

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Holt PhysicsChapter 5: Work and EnergyI.

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Section 5.1Work A. Definition of work 1. Work does not mean the same thing in Physics as it does in the everyday sense of the word. 2. Work is defined as a force causing a displacement.

Work = force x displacement W = Fd3. Work is NOT done on an object unless the displacement is greater than zero 4. The only forces that are considered to do work are those that are parallel to the displacement. 5. For this reason we use our trigonometric functions to calculate forces applied at an angle.

6. Note that is the angle between the applied force and the displacement. 7. Work is described in Newtons x meters (force x displacement). The unit of work is the Joule (J) 8. 1 Newton meter = 1 Joule 9. Work is a vector with both direction AND magnitude. This means WORK CAN BE NEGATIVE! 10. Negative work is most commonly used to slow an object down or decrease its velocity. II. Section 5-2: Energy A. Kinetic Energy 1

1. Kinetic energy is associated with an object in motion. 2. Kinetic energy depends on speed and mass

Kinetic Energy = mv23. Kinetic energy is a scalar and will use Joules as its standard unit. 4. It takes some work to change an objects kinetic energy either by changing the velocity or changing the mass. 5. This fact leads us to the Work-Kinetic Energy Theorem

WORK-KINETIC ENERGY THEOREM Wnet = KENet work = change in kinetic energy

6. Notice that Wnet = KE

and Wnet = Fnetd(cos) (from section 1) therefore, KE = Fnetd(cos)

B. Potential Energy 1. Kinetic energy is associated with motion potential energy is stored energy with the potential to do work given the right conditions. a. Examples: a ball at the top of a ramp, an arrow in a bent bow, a rocket on a launch pad 2. Gravitational Potential Energy is the result of an objects position relative to a gravitational field. (also measured in Joules)

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Gravitational Potential Energy = mass x gravity x height

Gravitational Potential Energy PEg = mgh

3. Gravitational potential energy is the result of

an objects positionrelative to some zero level. (p.178) This means that we compare an objects potential energy relative to a position where its potential energy would be zero. 4. Elastic Potential Energy can be stored in springs and elastic.

Elastic Potential Energy = spring constant x distance stretched or compressed

Elastic Potential Energy PEelastic = kx2

k = the spring constant and is different for every spring (weak springs have a small k, strong springs have a large k) and are measured in N/m. b. x = the distance the object is stretched or compressed in metersa.

III. Section 5-3: Conservation of Energy A. The First Law of Thermodynamics is that In any process, the total energy of the universe remains constant.Wikipedia This means that energy3

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can never be created or destroyed only converted into mass and back again. (E = mc2) B. Mechanical Energy (kinetic and potential energy) is often conserved if we neglect friction. C. Energy can change form (from potential to kinetic, to gravitational to elastic, but will always total up to be the same D. Sample Problem 5E on p.184 provides a good example E. Friction converts mechanical energy (movement) into non-mechanical energy (heat)ergo mechanical energy is NOT conserved in the presence of friction, but total energy is. Section 5-4: Power A. The rate at which work is done is called power

Power P = W/tPower = work / time

B. Since work (W) is defined as Force x distance we can

rewrite P = Fv (power = force x velocity) C. The standard unit for power is the watt D. Conversions: 1 watt = 1 joule/second 1 horsepower = 746 watts

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