1 / 21

Work, Energy, Power

Work, Energy, Power. What’s the difference?. Force is the agent of change Energy is a measure of change Work is a way of transferring energy from one system to another. What is work?. Work= force*displac. W=Fd

aminia
Download Presentation

Work, Energy, Power

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Work, Energy, Power

  2. What’s the difference? • Force is the agent of change • Energy is a measure of change • Work is a way of transferring energy from one system to another

  3. What is work? • Work= force*displac. • W=Fd • Only work if there is motion- if you push against a brick wall and it doesn’t move, you might be tired but you have done no work • Unit=Joule (unit of energy)

  4. Are they work? • No- no displacement • Yes- force=g and displacement=fall • No-why? • Yes- force from engines • Teacher pushes wall and becomes exhausted • Book falls off table to floor • Waiter carries large tray across restaurant at constant v • Starship Enterprise accelerates through space So what’s with the waiter???????

  5. Work = 0 • Work = 0 if: • No force • No displacement • force is perpendicular to displacement

  6. Power • Power= rate at which work gets done= work over time • P=W/t • Since W=Fd then P=Fd/t and d/t=v • P=Fv • Unit= J/s=watt (W) • Careful not to confuse unit W (watt) with concept W (work)

  7. Ex: Power • A mover pushes a large crate mass=75kg across the truck bed for a total distance of 6m. He exerts a steady force of 300N for 20s. What is his power output? • P=W/t P=Fd/t=(300N)(6m)/20s=90W

  8. Kinetic Energy • Energy of MOTION • K=1/2mv2

  9. Example… • Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s • KE = (1/2)*m*v2 • KE = (0.5) * (625 kg) * (18.3 m/s)2 • KE = 1.05 x105 Joules

  10. Potential Energy=U Energy an object has due to its position or configuration- stored energy that can be retrieved. Ex- height on a wave gives U, pulling back the string on a bow gives it U, compressing or stretching a spring gives U.

  11. Potential Energy: Energy of POSITION

  12. Gravitational Potential Energy: Ug • Potential energy due to position relative to surface of the earth • Ug=mgh • Unit = Joule

  13. Gravitational Potential Energy: Ugand Work done by gravity • Gravity can do + or - work depending on motion • Path independent- depends on height, not path taken • Wg=mgΔh • Where h is height above arbitrary 0 pt

  14. Examples • Physicsman (mass=60kg) scales a 40m tall rock face. What is his potential energy (relative to the ground)? • Ug=mgh=(60kg)(10N/kg)(40m)=24000J

  15. Mechanical Energy: U and K:Energy because of position or motion

  16. Total Mechanical Energy is CONSERVED PE ONLY PE + KE KE ONLY

  17. Take it 1 step further… • If physicsman (60kg) were to jump of the cliff (remember his U=24000J), what would his velocity be when he hits the ground? Think…U is transformed to K • At the top he has all U=24000J • At the bottom he has all K=1/2mv2 • Utop=Kbottom • 24000J=1/2(60kg)v2 • V=28m/s

  18. Work by Conservative vs. Nonconservative Forces • Conservative forces are path independent • Ex: gravity • Nonconservative forces depend on path • Ex: kinetic friction- longer path means more work

  19. Work and Energy • E=K+U • E=1/2mv2+mgh • Object’s mechanical energy is sum of kinetic and potential energies • Since U is relative to position, so is E • Wnc=ΔK+ΔU • Work done by nonconservative forces is sum of changes in K and U

  20. Conservation of Energy • Since E=K+U, if no nonconservative forces (friction for example) act on a system then mechanical energy is conserved • Ei=Ef • Ki+Ui=Kf+Uf

  21. Ex: conservation of energy • A ball of mass 2kg is gently pushed off the lab table, 5.0m above the floor. Find the speed of the ball as it strikes the floor • Ei=Ef or Ki+Ui=Kf+Uf • 0+mgh=1/2mv2+0

More Related