Fig. 3.1 shows a door and an automatic door-closer viewed from above. [Figure 3.1] When the door opens and closes, the hinge acts as a pivot. A girl opens the door by exerting a force P at point X. Force P is perpendicular to the surface of the door.

1. (a)(i)Fig. 3.1 shows that point X is a distance of 0.72m along the front of the door from the hinge. The force P is 25 N. Calculate the moment of force P about the hinge.[2]
2. (a)(ii)The door rotates about the hinge by 90°. The circumference of a circle of radius 0.72 m is 4.5m. Calculate the work done on the door by force P.[2]
3. (b)As the door opens, there is a force F on the door in the direction shown in Fig. 3.1. Although force F is larger than force P, the door rotates about the hinge. Explain why.[2]

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