Work, Energy, & Power Chapter 8

1. Work, Energy, & PowerChapter 8

2. Let’s start with WORK… • Work is only done if an object is displaced by the force, in the same direction as the force!

3. Work • Work is the process of changing the energy of the system • Units: Joules • 1,000 J = 1kJ (1kilo-Joule)

4. How do we do Work? • Changing the speed of an object • Catching a baseball • Applying brakes of your car • By doing work against another force. • Doing a pull-up • Bench press • Or simply getting up out of your chair W = F∆d

5. Practice Problems • Work is only done if the object is displaced in the same direction as the force.

6. Solutions • A: W = Fd = 100N x 5 m = 500J • B: W = Fd = [100N Cos (30°)] x 5 m = 433J • C: W = Fd = (mg) d = (15kg x 9.8m/s2) 5 m = 750J 100N 30°

7. Power • Power is simply the rate at which work is done. • The faster we do Work… the more powerful our action is • The slower we do that same Work… the less powerful our action is What makes the backhoe loader more POWERFUL?

8. Power • Power is simply the rate at which work is done • P = W / t • Units: Watt

9. From Work to Energy • CAUSE… WORK • EFFECT… ∆ENERGY

10. Homework • Read 8.1 - 8.2 • Complete Problems 1- 5, page 2 of packet

11. The Many Forms of Energy • Energy is the ability of an object to cause a change in itself or the environment • Thermal - Radiant • Nuclear - Gravitational (PE) • Chemical - Sound • Electrical - Electrochemical • Motion (KE)

12. Mechanical Energy • There are two types: • Gravitational Potential Energy • Kinetic Energy

13. Recall… Of Course! like any force, the gravitational force can cause an object to be displaced. We call this work, Gravitational potential energy Can Gravity Do Work? W = F∆d

14. Work and Gravitational Potential Energy • We know…W = Fd • Or in the specific case of gravity doing the work we know F=mg • So… W = (mg)d • So, we could say… W = Fd = mgh • You might recall… PE = mgh • Work = Fd = mgh = Gravitational Potential Energy (PE)

15. Gravitational Potential Energy (GPE) • Known as energy of position • Measured in Joules • GPE = mgh • Example: How much GPE does a 4500 kg roller coaster possess if it is poised on top of a 48 m high hill?

16. Kinetic Energy • Start with equation 7 • Vf2 = Vi2 + 2ad • Substitute F/m in for a (F=ma)

17. Kinetic Energy • Therefore • Vf2 = Vi2 + 2Fd/m • Solve for Fd • Vf2 - Vi2 = 2Fd/m • 1/2Vf2 – 1/2Vi2 = Fd/m • 1/2mVf2 – 1/2mVi2 = Fd

18. Work-Energy Theorem • Therefore • 1/2mV2 = Fd • Where 1/2mV2 is the Kinetic Energy (KE) • Where Fd is the Work (W) • Therefore W=Fd=1/2 m∆v2 = ∆KE

19. Kinetic Energy • Known as energy of motion • KE = ½ mv2 • Example: Same rollercoaster is traveling at 20.6 m/s. Kinetic Energy?

20. The Law of Conservation of Mechanical Energy • Energy can not be created or destroyed. It can only be transformed. • One of the “Big Three” Conservation Laws (the others being Mass and Momentum) • In other words, if you have 1,000 J of energy you can change it into other forms of energy without losing any of it. Every, single Joule is accounted for.

21. Transform: Change forms

22. You Make the Calculations:

23. ……..and these

24. Homework • Read 8.3-8.6 • (Mechanical, Potential, and Conservation of Energy) • Complete Problems • 1 (a, b, and c only), and 2-4, page 7 of packet