Work and Energy

1. Work and Energy How do we calculate kinetic and potential energy? • Key Concepts • Sample Problems • Practice worksheets

2. Main ideas • An object that has energy can do work. • Work is done when energy is transferred from one object to another or changed from one form to another.

3. CALCULATING ENERGY

4. ENERGY A BODY HAS BECAUSE OF ITS POSITION OR CONDITION P.E.= Ep = mgh G.P.E. = weight x height

5. EXAMPLE What is the p.e. of a book of mass 1.2kg, resting on a shelf 2 m above the ground? p.e. = mgh p.e. = 1.2kg x 10m/s/s x 2m p.e. = 12.0 N x 2 m p.e. = 24 N-meter or Joules

6. SAMPLE PROBLEM: A hiker weighing 680 N climbs 40 m up a hill. Calculate the gravitational potential energy gained at the top of the climb. Ep= mgh

7. P.E.= mgh Ep = 680 N x 40m = 27,200 N-m or 27.2 KJ

8. mv2 KE 2 KINETIC ENERGY (Ek) = ½ mv2 =

9. http://www.youtube.com/watch?v=BSWl_Zj-CZs&feature=related Tutorial- Calculating PE and KE

11. POTENTIAL ENERGY

12. ENERGY A BODY HAS BECAUSE OF ITS POSITION OR CONDITION P.E.= Ep = mgh G.P.E. = weight x height

13. POTENTIAL ENERGY Ep or p.e. is the potential energy an object has because of its height. “The higher an object is lifted, the greater its p.e.” “The greater the mass of an object the greater its p.e.”

14. 1. Calculate the p.e. of a 40 kg iron bar raised 1 m. (g = 10N/kg) 2. Calculate the p.e. of a 65 kg boulder resting at 22 m above the road. 3. A 200 g jar falls from a height 1.5 m onto the floor. Calculate the p.e. of the jar before the fall.

15. ANSWERS 1. Calculate the p.e. of a 40 kg iron bar raised 1 m. 400 N-m or 400J 2. Calculate the p.e. of a 65 kg boulder resting at 22 m above the road. 14,300 N-m or 14.3 KJ 3. A 200 g jar falls from a height 1.5 m onto the floor. Calculate the p.e. of the jar before the fall. 3 J

16. KINETIC ENERGY

17. KINETIC ENERGY • Kinetic energy Ek or k.e. is the • energy that an object has because it • is moving • The unit of measure for k.e. is joule • The greater the mass of a moving • object, the greater the k.e. of the • object

18. What is the Ek of an arrow weighing 0.025 kg traveling at 25m/s?

19. What is the Ek of an arrow weighing 0.025kgtraveling at 25m/s? Answer: k.e. = ½ mv2 = ½ x 0.025kg x 25m/s2 = ½ x 0.025kg x 625m/s = (0.5) 15.625 = 7.8125 Nm but 1 Nm = 1J = 7.8125 J

20. Work and Energy How do we calculate kinetic and potential energy? • Key Concepts • Sample Problems • Practice worksheets

21. Potential Energy = Weight x Height (P.E. = w x h) or Ep= mgh Kinetic Energy = ½ Mass x Velocity2 (K.E.= 1/2mv2) weight = mg Work= F x d Units: Energy = joules (J) Weight = newtons (N) Mass = kilograms (kg) Velocity = m/s Height = meters (m) Gravity constant = 9.8m/sec/sec Work = joules 1km = 1000m 1kg= 1000g 1hr= 3,600sec

22. GPE = w x h or GPE = mgh • SUMMARY OF CONCEPTS: • If h is doubled, the GPE is doubled. • If H is tripled, the GPE is also tripled. • If speed (velocity) is doubled (2x), the Kinetic • Energy quadruples (increases 4x or 22). • If speed (velocity) is tripled (3X), the KE increases • 9xor 32.

23. CONSERVATION OF ENERGY http://www.youtube.com/watch?v=o_5oYuDY2qM&NR=1 Intro

24. Law of CONSERVATION OF ENERGY “Energy cannot be created nor destroyed. Energy is converted from one form to another.”

25. PE = KE

26. WORK K.E. P.E.

27. WORK AND ENERGY • 1.PE = KE • 2. Work = Energy • Work = PE gained = KE • Work = KE