A 90 kg man and his 18 kg daughter stand on opposite ends of a 4.3 m long wooden

Question

A 90 kg man and his 18 kg daughter stand on opposite ends of a 4.3 m long wooden plank with a mass of 15 kg. (a) If the system is taken to be the man and the daughter, how far along the plank from the daughter is the centre of mass of that system (in m)? (b) If the system is taken to be the man, the daughter, and the wooden plank, how far along the plank from the daughter is the centre of mass of that system (in m)? If you want to balance the wooden plank with the man and daughter on it, would you place the pivot directly under the centre of mass you found in part (a) or in part (b)? (c) The pivot should be placed under the centre of mass found in part (a). . The pivot should be placed under the centre of mass found in part (b).

Explanation / Answer

a) Xcm = (18*0+90*4.3)/(18+90) = 3.58 m (ans)

b) Xcm = (18*0+90*4.3+15*2.15)/(15+18+90) =3.41 m (ans)

c) The pivot should be placed under the centre of mass found in part b

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