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Physics

Physics. Session. Work, Power and Energy - 1. Session Objectives. Session Objective. Work done by constant force Work done by variable force Kinetic Energy Work-Energy Theorem Conservative and non-conservative forces Potential Energy and Total Mechanical Energy.

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Physics

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  1. Physics

  2. Session Work, Power and Energy - 1

  3. Session Objectives

  4. Session Objective • Work done by constant force • Work done by variable force • Kinetic Energy • Work-Energy Theorem • Conservative and non-conservative forces • Potential Energy and Total Mechanical Energy

  5. Work done by constant Force DisplacementS Component of force in the direction of displacement = Fcosq Work done = (Fcosq) S =FS cos

  6. Work by Varying Force Total work done is sum of all terms from xi to xf

  7. Work by Varying Force

  8. Work done on a particle between any two points is independent of the path taken by the particle. Force of gravity & spring Force Frictional Force & Viscous force Work done on a particle between any two points depends on the path taken by the particle. Conservative Forces Non-Conservative Forces

  9. Illustration of principle Conservative Forces : Work done will be same Non-Conservative Force : Work done will be different

  10. Kinetic Energy Energy associated with the motion of a body. Nature : scalar Unit : joules(J)

  11. Work Energy Theorem Work done by Total work done by all the forces acting on a body is equal to the change in its kinetic energy. Conservative forces (Wc) Non-Conservative forces (Wnc) External forces (Wext)

  12. Conservative Forces & Potential Energy Work done by a conservative force equals the decrease in the potential energy. Ui can be assigned any value as only Uis important

  13. Conservative Force: Spring force Force varies with position. Fs=-kx [k :Force constant] Work done in compression/extension of a spring by x = PE stored in the spring

  14. Class Test

  15. Force acting on a body is (a) dependent on the reference frame (b) independent of reference frame (c) dependent on the magnitude of velocity (d) None of these Class Exercise - 1 Solution : A specific force is external in origin and so is independent of reference frame. Hence answer is (d).

  16. A particle moves from a point to another point during which a certain force acts on it. The work done by the force on the particle during displacement is: (a) 20 J (b) 25 J (c) zero (d) 18 J Class Exercise - 2 Solution : Hence answer is (c)

  17. A force F = (a + bx) acts on a particle in the x-direction where a and b are constants. The work done by this force during a displacement from x = 0 to x = d is • zero (b) • (c) 2a + bd (d) (a + 2bd)d Class Exercise - 3 Solution : Force is variable. Hence answer is (b)

  18. A block starts from a point A, goes along a curvilinear path on a rough surface and comes back to the same point A. The work done by friction during the motion is: • positive (b) negative • (c) zero (d) any of these Class Exercise - 4 Solution : Friction always acts opposite to the displacement. Hence answer is (b).

  19. The work done by all forces on a system equals the change in • total energy • (b) kinetic energy • (c) potential energy • (d) None of these Class Exercise - 5 Solution : Statement characterizes kinetic energy. Hence answer is (b).

  20. A small block of mass m is kept on a rough inclined plane of inclination q fixed in an elevator. The elevator goes up with a uniform velocity v and the block does not slide on the wedge. The work done by the force of friction on the block in time t will be • zero (b) mgvtcos2q • (c) mgvtsin2q (d) mgvtsin2q Class Exercise - 6

  21. f vt mg Sin q mg q Solution f = mg sinq as block does not slide. Displacement d = vt Angle between d and f = (90 – q) W = fd cos(90° – q) = mgvt sin2q

  22. A force (where K is a positive constant) acts on a particle moving in the x-y plane. Starting from the origin, the particle is taken along the positive x-axis to the point (a, 0) and then parallel to the y-axis to the point (a, a). The total work done by the force on the particle is: • –2Ka2 (b) 2Ka2 • (c) –Ka2 (d) Ka2 Class Exercise - 7

  23. y 2 a x O 1 a Solution First path: y = 0 Second path: x = a Displacement along x = 0

  24. Under the action of a force, a 2 kg body moves such that its position x as a function of time is given by where x is in metre and t in seconds. The work done by the force in the first two seconds is: (a) 1,600 J (b) 160 J (c) 16 J (d) 1.6 J Class Exercise - 8

  25. Solution Hence answer is (c)

  26. There is a hemispherical bowl of radius R. A block of mass m slides from the rim of the bowl to the bottom. The velocity of the block at the bottom will be: Class Exercise - 9

  27. PE = mgR R 1 KE = mv 2 2 Solution PE = mgR Hence answer is (b)

  28. A block of mass 250 g slides down an incline of inclination 37° with uniform speed. Find the work done against the friction as the block slides down through 1.0 m. (a) 15 J (b) 150 J (c) 1.5 J (d) 1500 J Class Exercise - 10

  29. F=mN N Mg sin37o Mg cos37o 37o mg Solution As the block slides with uniform speed, net force along the incline is zero. Mg sin37° = m N \ Work done by gravity = Work done against friction = Mg sin37° x s = 1.5 J Hence answer is (c)

  30. Thank you

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