Question

A trolley of mass 20kg is at rest on smooth horizontal surface.A boy throws horizontally one by one 5kg sand balls with 20m/s towards trolley.These sand balls stick with trolley.How many sand balls should stick to the trolley so that the trolley will move with 10m/s?[Assume that sand ball moved in horizontal direction]

Answer 1

Answer:

after 4 balls throw the trolley will move with speed 10 m/s

Explanation:

As we know that there is no external force on the system of trolley + ball

So after first ball throw we have

$m_1v_1 = (m_1 + m_2)v$

$5(20) = (20 + 5) v$

$v = 4 m/s$

now after second ball throw

$5(20) + 25(4) = (5 + 25) v$

$v = \frac{200}{30} = \frac{20}{3} m/s$

Similarly after Nth ball throw we have

$N(5 \times 20) = (20 + 5N)10$

$100 N = 200 + 50 N$

$N = \frac{200}{50}$

$N = 4$

So after 4 balls throw the trolley will move with speed 10 m/s

#Learn

Topic : Momentum conservation

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