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Energy

Explore the concept of energy, including its forms and conservation, mechanical energy, work, power, and their relationships. Learn how to calculate and understand the various aspects of energy in this comprehensive guide.

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Energy

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  1. Energy The Currency of the Universe

  2. Energy in General • Energy is a quantity that can cause change in a body or system. • Energy is a scalar quantity measured in Joules=N m = kg m2/s2 • Energy is expressed in different forms and can be transformed from one form to another without loss. • Energy is conserved for a single body or system when there are no outside forces

  3. Mechanical Energy Mechanical energy is conserved when there are no outside forces or when the only outside forces are due to gravitational or elastic forces.

  4. Mechanical Energy (ME) • Kinetic energy (KE) is the energy due to an object’s motion =1/2 mv2 • Gravitational Potential Energy = the energy stored in the magnetic field between Earth and another body =mg Dh • Elastic potential energy = energy stored by stretching spring or other elastic object = ½ k Dx2 • ME conserved for object or system when there are no outside forces or when the outside forces are gravity, a spring or some other conservative force.

  5. Check question Each of the balls to right has a speed of 1 m/s and a mass of 2 kg. The directions of their velocities are indicated by the arrows. What is the total kinetic energy of the system of balls?

  6. Check Questions: Some Energy Math • A car traveling at 40 mph has a certain kinetic energy. How much more KE does it have when it travels at 80 mph? • A firefighter has a certain gravitational PE when he climbs half way up a ladder. How much more PE will he have when he is at the top of the ladder? • An elastic ball stores a certain amount of PE when it is compressed 1 mm. How much more PE will it store when it is compressed 2 mm?

  7. Work (W) • Work is a force that changes the energy of a body or system. Scalar w/ same units as energy. • Work = force x distance object moves in direction of force = Fd(cos q) • Work is positive if force is in direction of motion and increases energy. • Work is negative if force is in opposite direction of motion and decreases energy. • Force and force components perpendicular to motion DO NO WORK

  8. Work and Energy Relationships Mechanical Energy Kinetic Energy Elastic Potential Energy Gravitational Potential Energy Positive Work Negative Work W=DKE + DPEg + DPEE

  9. Check Question • Mr. DeVoe lifts up a trash can and moves it to the curb every Thursday night. State whether work is done by Mr. DeVoe’s lifting force and if the work is positive or negative. • Picking can up • Moving can horizontally at constant speed • Slowing can down • Lowering can

  10. Power (P) • Power is the rate at which energy is used. • If machine A has twice the power of machine B, it can get the same amount of work done in half the time or twice the amount of work done in the same time. • Power = Work / time for work to be done = W/t • Scalar quantity with units of Joules/ second =Watt

  11. Check Question • A 400 N student climbs a flight of stairs that is 10 m tall. The student does this climb in 10 seconds. • How much work does the student do during this climb? • What type of energy does the work change? • What is the power of the student?

  12. Expressions of Mechanical Energy • Kinetic Energy = the energy expressed as an object’s motion (KE = ½ mv2) • Doubling the speed means… • Gravitational Potential Energy= Energy stored due to an object’s height above a reference level. (PE gravitational = mg Dh) • Elastic or Spring Potential Energy= Energy stored in the stretch or compression of an elastic object (PE elastic=1/2 kDx2) • Internal energy (U) is NOT mechanical energy

  13. Mechanical Work • Work is done when an outside force changes the energy of an object or system. • Work = force x distance object moves in direction of force. • W=Fdcosq • No work done if no motion or force is perpendicular to motion • Work has same units and is scalar like energy • Work on vs. Work by, + vs. - work

  14. Work-Energy Theorem • Work-kinetic energy theorem • Work = change in kinetic energy • True if no change in height or elastic displacement • Work-mechanical energy theorem • Work= DKE +D PE g +D PE e • True if no change in temperature • First law of Thermodynamics • Work + Heat Flow = DKE +D PE g +D PE e + DU • Always true

  15. Mechanical Energy • Mechanical Energy is the ability to cause changes in velocity, elevation or how much an elastic material is compressed or stretched. • Scalar quantity w/ units of = Joules = Nm= kg m2/s2 • Mechanical energy can switch from one form to another w/o loss • Conserved for single objects when there are no outside forces or when the only outside forces are gravity and elastic forces. • Not conserved when friction or air resistance act

  16. Power • Rate at which work is done or changed into another form of energy • Power = work / time • Units of watts or Joules / second, scalar quantity • More powerful machine • Gets same work done in less time • Gets more work done in same time

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