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Energy

Energy. Work. Work is a transfer of energy In order to do work on an object, you must increase the energy within it Any type of energy can do work Let’s look at a few…. Work. Kinetic Energy. Energy is the ability to do work Kinetic Energy ( KE ) is the energy of motion

ignatius-dayton
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Energy

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  1. Energy

  2. Work • Work is a transfer of energy • In order to do work on an object, you must increase the energy within it • Any type of energy can do work • Let’s look at a few…

  3. Work

  4. Kinetic Energy • Energy is the ability to do work • Kinetic Energy (KE) is the energy of motion • More speed means more KE

  5. Kinetic Energy

  6. Potential Energy (GPE) • Potential Energy is storedenergy • Gravitational Potential Energy (GPE) is when the energy is stored in its position (height) • The higher an object goes, the more GPE • (the faster the speed it will have when it hits the ground)

  7. Potential Energy (EPE) • The other type of Potential Energy we will look at is Elastic Potential Energy (EPE) • Instead of height, the energy is stored by stretching an object. • More stretching means more EPE • ex. rubber band, spring

  8. Potential Energy

  9. Heat • Heat energy is the energy created by friction • This could be from scraping, rubbing or deformation

  10. Work Examples • Let’s look at the following scenarios: • Work or not? • Lifting a box above your head • Holding that box there for 2 hours • Sliding a box across a frictionless surface at constant speed work not work not work

  11. Energy Equations: Work • Work is the product of the force applied in the directionof motion and the distance it is applied • When the force and the movement are parallel, work is simply Force (F) θ

  12. Energy Units • Notice: from the work formula, energy units are a combo of Force (Newtons) and distance (m) or Newton•meters(N•m) • The SI units for energy are Joules (J). • So, one Joule is equal to 1 Newton•meter.

  13. Energy Equations: GPE • For GPE, we still have force x distance, but this time the force is the objects weight, mg • This gives us the equation: • We use h instead of d since it will always be height for GPE F=mg m

  14. Energy Equations: KE • Now let’s throw a block • The work is done while the block is being accelerated by the hand a distance of d

  15. Energy Equations: KE • This time the force is simply ma • So, the work done is: • Quick time warp back to acceleration equation:

  16. Energy Equations: KE • Let’s substitute: • The normal equation assumes starting from rest (vi = 0):

  17. Energy Equations: heat • When pushing a block at constant speed across a surface, the friction force is turned into heat • Since added force is only working against friction (no a), all of the work done on the block is then turned into heat f

  18. Energy Equations: heat • Remember that d is only during the friction

  19. Energy Equations: EPE • EPE is trickier than GPE • force changes depending on how much you stretch the object • This force depends on both the distance stretched (x) and a spring constant (k) • This equation is known as Hooke’s Law

  20. Energy Equations: EPE • This k comes from how much force is needed to stretch a spring per a certain distance • What is the k for this spring?

  21. Energy Equations: EPE • Since the force at the beginning of the stretch is different than the end, we use an average to calculate the EPE: • Since we usually start the stretch from rest:

  22. Power • In physics, power just means the rate of doing work • So, faster work means more power • The units come out to Joules per second. • We call this a Watt (W)for short

  23. Check Yourself Go to pg. 445

  24. Conservation of Energy • Energy cannot be created nor destroyed, but only changed from one form to another • What does this mean?

  25. Conservation of Energy • All of the energy that you start with… • you end with! • initial energy = final energy • Total energy at top • equals • Total energy at bottom • Total energy anywhere

  26. Conservation of Energy • All of the energy that you start with… • you end with! • initial energy = final energy • Total energy at top • equals • Total energy at bottom • Total energy anywhere

  27. Conservation of Energy All GPE GPE andKE All KE

  28. Conservation of Energy Problems • Identify type of energy at beginning and end • Full law in equation form: • For most problems, many are zero

  29. Conservation of Energy: Example • Rolling down a hill from rest • Top (initial): all GPE • Bottom (final): all KE • Left with: • or:

  30. Conservation of Energy: Example • A bow is used to shoot a .050 kg arrow into the air. If the average force used to draw the bow is 110 N and the bow is drawn .50 m, how fast is the arrow moving when it has risen 35 meters above the bow? • (Assume air resistance is negligible) • Define: • initial : • and • final : What type of energy is it? (work) when bow is drawn (KE & GPE) when arrow is at 35 m

  31. Conservation of Energy: Example Write out CoEeqn and cross out missing E’s moving at rest start at h = 0 goes higher finding through work (no k) nothing stretched/pressed no air resistance

  32. Conservation of Energy: Example rewrite and expand solve for v

  33. Conservation of Energy: Example plug and chug

  34. Time to practice Turn to pg. 456

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