On an ice skating rink, a girl of mass 50 kg stands stationary, face to face with a boy of mass 80 kg. the children push off of one another, and the boy moves away with a velocity of +3 m/s. what is the final velocity of the girl? –1.9 m/s +1.9 m/s –4.8 m/s +4.8 m/s

- 4.8 m/s option (c)

Explanation:

Let the girl moves with velocity u.

mass of girl, m = 50 kg, u = ?

mass of boy, M = 80 kg, U = + 3 m/s

Use the conservation of momentum

momentum before pushing = momentum after pushing

0 = m x u + M x U

0 = 50 x u + 80 x 3

- 240 / 50 = u

u = - 4.8 m/s

work: w = fd (given)

work done by chet   =50*1 = 50 j

work done by mika =40*2 = 80 j

work done by sara =30*1.5 = 45 j

work done by bill =60*0.5 = 30 j

students in order from the greatest amount of work done to the least is mika> chet> sara> bill

hence option (b) is correct.

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