Suppose the pendulum bob in Figure 8-20 has a mass of 0.25kg. (a). How much work

Question

Suppose the pendulum bob in Figure 8-20 has a mass of 0.25kg.

(a). How much work does gravity do on the bob as it moves from point A to point B?

(b). From point B to point A?

(c). How much work does the string do on the bob as it moves from point A to point B?

(d). From point B to point A?

Figure 8-20: Angle between point B and point A is 35 degrees. Point B is when the bob is stationary and hangs straight down (perpendicular to a horizontal surface). Point A is when the bob swings 35 degrees to the right of point B. Length of string which pendulum bob is attached is 1.2 m long.

Explanation / Answer

Thanks for describing the picture... helps a lot. Too many people post questions like this and dont include any of the info in the pic.

(a) work done by gravity is always - m g (change in height)

in your case the change in height is negative (downward) and equal to L(1-cos@) =

= 1.2 * (1 -cos35) = 0.21702 meters

So

work done = 0.25 * 9.80* 0.21702 = 0.5317 Joules (positive... gravity makes it go faster)

(b) same amount but negative, i.e. -0.5317 Joules (negative because gravity slows it down)

(c) the work done by string is zero, because the tension acts perpendicular to the motion. The tension does not affect the speed of the pendulum... only gravity is responsible for changing its speed.

(d) work done by string is still zero. The force of the string is always perpendicular to the motion.

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