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Energy, Work, and Power

Unit 3. Energy, Work, and Power. Energy is the capacity for an object to do work For example, when a car moves, the engine performs work to get the car going.

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Energy, Work, and Power

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  1. Unit 3 Energy, Work, and Power

  2. Energy is the capacity for an object to do work For example, when a car moves, the engine performs work to get the car going. There are many different types of energy, including: electrical, kinetic, gravitational potential, and elastic potential to name a few. A more complete list can be found on p. 124 Types of Energy

  3. An energy transformation occurs whenever energy changes from one form into another. Examples of this would be a ball being held above the ground (gravitational potential) and then being released to fall to the ground (kinetic). Energy Transformation

  4. Work

  5. This is the energy transferred to an object The object must move a distance as a result of the force applied Does it matter what direction the object moves?? Work

  6. Work requires a force Work requires a distance This leads us to say: WαF and WαΔd This gives us: W = F Δd The units are Newton Meters (Nm) or, more commonly, Joules (J) How to calculate work

  7. A 600 N force is applied by a person to a dresser that moves 2 m. Find the work done if the force and the displacement are • Parallel • At right angles • Oppositely directed Examples

  8. A horse pulls a barge along a canal with a rope in which the tension is 1000N. The rope is at an angle of 10° with the towpath and the direction of the barge • How much work is done by the horse in pulling the barge 100m? • What is the net force on the barge?

  9. For there to be work, Remember!!!!

  10. Any force applied in the same plane causes work to be done If the force makes the object increase in speed, then it is positive work If the force makes the object slow its speed, then it is negative work. These forces are called Dissipative Forces All friction is negative work. Positive and Negative Work

  11. When we lift something up, we do work, why is this? • When we look at this type of work, we still must look at the force we are working with • Fg = mg • This lead to the following • W = Fgd • W = mgd Gravity

  12. A bag of groceries of mass 8.1 kg is raised vertically without acceleration from the floor to a counter top, over a distance of 92 cm. Determine • The force needed to raise the bag without acceleration. • The work done on the bag against the force of gravity Example

  13. Mechanical Energy

  14. There are 2 types of mechanical energy • Gravitational Potential Energy • Kinetic Energy • Gravitational Potential Energy • This is energy that can be used to do work at a lower level • Kinetic Energy • This is the energy of motion Mechanical energy

  15. To hit a nail with a hammer, what must you do? • By lifting the hammer, Δh, you also need to apply a force. • The height is measured from a starting point or equilibrium position. • The force is found by lifting the mass against gravity • Ep = FΔh • Ep = mg(-) Determining Potential energy

  16. example • Assume that a 59 kg pole vaulter must raise their center of mass from 1.1 m off the ground to 4.6 m off the ground. What is the jumper’s gravitational potential energy at the top of the bar relative to where the jumper started to jump? • Ep = mgΔh • Ep = (59)(9.81)(4.6-1.1) • Ep = 2.0 x 103 J

  17. Grain Auger Pile Drivers Hydro Dams We use this in Red Lake everyday Applications of mechanical energy

  18. If you are interested in how the formula is generated, see p. 134 Kinetic energy is the energy of motion, so what do we need? Ek = ½ mv2 Determining kinetic energy

  19. example • Determine the amount of kinetic energy of a 48 g dart travelling at a speed of 3.4 m/s. • Ek = ½ mv2 • Ek = ½ (.048)(3.4)2 • Ek = 0.28 J

  20. Law of conservation of energy

  21. We know that there are many types of energy transformations When energy changes forms, energy is conserved What does this mean? Energy is never lost, it just changes form Energy conservation

  22. example

  23. An object which weighs 10 N is dropped from rest from a height of 4m above the ground. When it has free-fallen 1 meter, its total mechanical energy with respect to the ground is_______

  24. An archer needs to exert 275 N of force to pull her bow string back 0.500m. If the mass of the arrow is 3.00 , what is the final speed of the arrow?

  25. A skier glides down a frictionless hill of 100 meters, the ascends another hill, of height 90 meters, as shown in the figure below. What is the speed of the skier when it reaches the top of the second hill? 100m 90 m

  26. A comparison of the amount of energy put into a system compared to the amount of energy output in a system %E = Eout/Ein Efficiency

  27. Energy input is the amount of energy that is being supplied by the person doing the work Energy output is the amount of energy that would be created if it could be vertically released Energy Input and output

  28. Power is the rate of doing work or transferring energy This is a scalar quantity P = W/ΔT P = ΔE/ΔT Power

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