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Work, Power and Energy

Work, Power and Energy. Physics 11 Mrs. Kay. Work. The product of the force exerted on an object and the distance the object moves in the direction of the force. W=Fcos Ө d Units = J (joule)= N (newton) x m (metre) Work is done only if an object moves. Force vs. Displacement.

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Work, Power and Energy

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  1. Work, Power and Energy Physics 11 Mrs. Kay

  2. Work • The product of the force exerted on an object and the distance the object moves in the direction of the force. • W=FcosӨd • Units = J (joule)= N (newton) x m (metre) • Work is done only if an object moves

  3. Force vs. Displacement • The area under a force displacement graph equals the work done.

  4. If movement is in same direction • Holding objects doesn’t count as work, because no movement. • If perpendicular to movement cos(90o)=0, so no work. • If parallel to movement cos(Oo) =1, so W=Fd

  5. Problem: • How much work is done by a weight lifter is she holds a 100N barbell above her head? Answer: W =FcosӨd =100N cos(90o)(Om) = 0 J

  6. Problem 2: 2. A man carries a 50N box on his shoulder and walks 30m. How much work does he do on the box? Answer: W =FcosӨd =50N cos(90o) 30m =O J

  7. Problem 3: • A girl exerts a force of 35N at a 35o angle to pull her wagon 25m down a street. How much work does she do on the wagon? Answer: W =FcosӨd =(35N)(cos35o)25m = 717J

  8. Problem 4: • A superhero stops a runaway bus by exerting a force of 200N against the motion of the bus for 12m. How much work does he do on the bus? Answer: W =FcosӨd =(200N)(cos 180o) (12m) =-2400J (negative because in opposite direction of motion)

  9. Remember: • If force and motion are horizontal and in the same direction you can use: • W=Fd

  10. When No force is done

  11. Work against gravity • When there is a vertical and horizontal component, the displacement is equal to the height or vertical component. It does not depend on the incline. • Ex: like a stairs or escalator question. • Use W=mgh, where h is the height that the object has traveled.

  12. Problems: • A 75kg firefighter climbs up a flight of stairs 10m high. How much work is required? • A 900N crate rests on the floor. How much work is required to pull it 6m up a 35o incline? • A girl holds a 15kg bag above her head. How much work did it take for her to lift it up to 2.2m?

  13. Answers: • W=mgh, (75kg)(9.8m/s2)(10m)=7350J • Vertical = sin(35) x 6 = 3.44m, so… W=Fd=(900N)(3.44m) =3097J • W=mgh = (15kg)(9.8m/s2)(2.2m)=323.4J

  14. Practice: Day One: • Pg 199 #1-3 • Pg 202 # 5-7 • Pg 213 1-3 and 5 Day Two: • Pg 213 # 7-11

  15. Power • The rate of doing work • P=W/t • Units: watt (w) = 1 Joule of energy transferred per second.

  16. Problems: • Pg.203 #9-11 • Pg.214 # 12-13,15,18

  17. Forms of Energy • Kinetic energy: found in moving objects • Potential energy: stored energy. It is converted into kinetic energy. Can have many forms within it. (elastic, gravitational, chemical…) • Energy is measured in Joules (J)

  18. Kinetic Energy • The kinetic energy of an object is given by the equation: Ek=1/2 mv2 • It is proportional to the mass and square velocity of the object. • The heavier and more quickly the object is moving, the greater the kinetic energy

  19. If there is a constant acceleration, and therefore a constant net force, and we assume the object is originally at rest, then W=Ek, or work is equal to kinetic energy

  20. Work-Energy Theorem • Not all objects start at rest. • They may begin with work exerting on them, giving them an original Ek. • W-E Theorem states: The change in the kinetic energy of an object is equal to the net work done on it. Wnet = Ekf – Eki = Ek

  21. W-E theorem • If net work is positive then Ek increases. Ex: Pitch a ball forward, the ball and force are in same direction so net work is positive.

  22. If net work is negative then Ek decreases. Ex: catch the ball with mit, the balls motion if forward, but the mitt exerts a force opposite to the motion on the ball because the ball is slowed to zero Ek. The mitt exerts negative work on the ball.

  23. Potential Energy • As you throw up a block, it loses kinetic energy (b/c it is slowing down), however it is gaining gravitational potential energy

  24. Gravitational Potential Energy • Any time an object is thrown up, it must come down. Its downward motion is losing kinetic energy, but gaining potential energy • Ep=mgh (is only valid if g is constant) • Enet= Ek + Ep

  25. Elastic Potential Energy • Energy can be stored in bending or stretching objects (like springs or elastics) • k= spring’s constant • x= distance spring is stretched

  26. Practice: • Pg.221 #1-3 (Ek) • Pg.224 # 5-7 (Ep) • Pg. 237 #1-4 (Problems)

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