**Exercise 9-8: **
A teeter-totter is 20 kg in mass. Assume it is a uniform plank of
length 2 m. The pivot point is placed 0.2 m from the midpoint. What
weight must be added to the short end to balance the teeter-totter?

(Hint: Take moments about the pivot - the uniform plank can be assumed to
be a single weight concentrated at its midpoint. Can you justify this?)

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