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A ball with a mass of #9 kg# moving at #7 m/s# hits a still ball with a mass of #15 kg#. If the first ball stops moving, how fast is the second ball moving?

Answer 1

Charlotte Aaron

#4.2# #m/s# towards the direction the first ball was going previously.

I'm Considering the Collision is Elastic.

According to the Law of Conservation of Momentum,

#m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2# [The total momentum of

a system should be constant after a collision]

So, Substituting the values,

#9 xx 7 + 15 xx 0 = 9 xx 0 + 15 * x# [Let the required velocity be #x#.]

#rArr 15x = 63#

#rArr x = 63/15 = 4.2#

The velocity is positive, so the second ball will head towards the direction the first ball was initially going.

Hence explained.

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Answer 2

Nathan Ashcroft

The second ball will move at 4 m/s after the collision.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.