3. TopicSlidesMinutes 1 Displacement 927 2 Vectors 1339 3 Kinematics 1339 4 Graphs 1030 5 Energy 1030 6 515 7 Springs 412 8 Shadows 39 9 Field of Vision 721 10 Colors 39 11 Concave mirrors 721 12 Convex mirrors 412 13 Refraction 515 14 Lenses 1030 15 Optical Power 618 Power

4. Click Power is the rate of doing work. It tells us how fast work is being done. Power is important in order to establish the “speed” with which a given force acts. The formula for power is: Velocity Where: P = power in watts (W) F = force in newtons (N) t = time in seconds (s) v = velocity in m/s

6. A 20 kg object is pushed 15 m along a horizontal frictionless surface by a horizontal force of 8 N. If the work was done in a time of 12 s, calculate: Click a) The work done. 120 J Click b) The power output. 10 W c) What becomes of the work done? It is lost to overcome friction (in the form of heat and sound) Click Power Slide: 6. 16. 1 Note that we assume the object is moved at constant velocity.

7. What power is required to raise a mass of 40 kg a distance of 5 m in a time of 2 s? Click Power Slide: 6. 26. 2

8. A 2 000 kg elevator is raised a distance of 18 m in a time of 40 s. Determine: Click a) The work done. 3.6 x 10 5 J Click b) The rate of doing work. 9 x 10 3 W c) What becomes of the work done? It is gained by the elevator in the form of potential energy. Click Power Slide: 6. 36. 3

9. If 700 W of power is needed to keep a boat moving through water at a constant speed of 10 m/s, what is the magnitude of the force exerted by the motor of the boat? Click Power Slide: 6. 46. 4

10. Betty uses the hoist illustrated on the right to raise a mass of 200 kg. If she can pull the rope 5.0 metres in a time of 10 seconds, what is Betty’s power output? Note: Disregard friction. Click A) 17.0 W B) 100 W C) 163 W D) 980 W I.M.A. = 6 (6 ropes) Power Slide: 6. 56. 5

11. … and good luck!