Holt Physics—Chapter 5: Work and Energy

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Holt Physics—Chapter 5: Work and Energy Author: alan price Last modified by: alan price Created Date: 8/28/2007 3:10:00 PM Company: Denver Academy Other titles:
Holt Physics—Chapter 5: Work and Energy Price

I. Section 5.1—Work 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 = Fd

3. 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.







Insert Fig 5-2



6. Note that Θ 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. Kinetic energy is associated with an object in motion. 2. Kinetic energy depends on speed and mass



Kinetic Energy = ½mv2

3. Kinetic energy is a scalar and will use Joules as its standard unit. 4. It takes some work to change an object’s 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 = ∆KE Net work = change in kinetic energy

6. Notice that Wnet = ∆KE and Wnet = Fnetd(cosΘ) ( (from section 1) therefore, ∆KE = Fnetd(cosΘ)

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 object’s position relative to a gravitational field. (also measured in Joules)

Gravitational Potential Energy PEg = mgh Gravitational Potential Energy = mass x gravity x height



3. “Gravitational potential energy is the result of an object’s position…relative to some zero level.” (p.178) This means that we compare an object’s 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 PEelastic = ½kx2 Elastic Potential Energy = ½spring constant x distance stretched or compressed

a. 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 meters



(insert fig. 5-8)

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 energy 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. IV. Section 5-4: Power A. The rate at which work is done is called power

Power P = W/Δt Power = 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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