A 40-g ball travelling to the right at 30 cm/s collides head on with an 80-g ball that is at rest. If the collision is perfectly elastic, find the velocity of each ball after collision
A. the first ball is going to the right at 10m/s while the other is going to the left at 20m/s
B. the first ball is going to the left at 10m/s while the other is going to the right at 20m/s
C. the first ball is going to the left at 20 m/s while the other is going to the right at 10 m/s
D. the first ball is going to the right at 10 m/s while the other is going to the left at 10 m/s

Question

A 40-g ball travelling to the right at 30 cm/s collides head on with an 80-g ball that is at rest. If the collision is perfectly elastic, find the velocity of each ball after collision
A. the first ball is going to the right at 10m/s while the other is going to the left at 20m/s
B. the first ball is going to the left at 10m/s while the other is going to the right at 20m/s
C. the first ball is going to the left at 20 m/s while the other is going to the right at 10 m/s
D. the first ball is going to the right at 10 m/s while the other is going to the left at 10 m/s

anonymous · Answer

For perfectly elastic collision, the kinetic energy and linear momentum are conserved.  Can you try to do the problem by yourself now??

anonymous · Answer

sorry, im not too good with Physics so pardon my indulgence. I tried it but was not getting the answer...

anonymous · Answer

No problem.  Tell me your Conservation of kinetic energy and conservation of linear momentum equation and I will corret them if they are wrong. :)

anonymous · Answer

*correct

anonymous · Answer

Check your equations from this link http://en.wikipedia.org/wiki/Elastic_collision#One-dimensional_Newtonian

anonymous · Answer

Do you mean the energy before collision is equal to the energy after?

anonymous · Answer

that is, m1u1 + m2u2 = m1v1 + m2v2

anonymous · Answer

Yes

anonymous · Answer

Now kinetic energy equation.  Also  What is u2 in our case

anonymous · Answer

okay, here it is 0 because the block is at rest.

anonymous · Answer

K.E = 1/2mv^2

anonymous · Answer

I meant conservation of kinetic energy equation
Yes u2 is 0

anonymous · Answer

1/2mv^2 = mgh ???

anonymous · Answer

Sorry. It is not right.  Think.  Initially only one object is moving with velocity v1 and finally the 2 blocks are moving with velocity v' . Now tell me again.  If you are still confused, then tell me.  I will guide you.

anonymous · Answer

1/2m1v^2 = (m1+m2)gh ???

anonymous · Answer

No mgh is potential energy.  It should be
$$(1/2) m_1v^2 = (1/2)m_1 v_f^2+ (1/2)m_2v_f^2$$
Hope you understand it now.  Any doubts on how I got this?

anonymous · Answer

oh yeah..

anonymous · Answer

Now substitute the given quantities in both equation and solve them.

anonymous · Answer

Any problem, post it here

anonymous · Answer

NOTE: Make sure you convert the given quantities in SI units before substituting them.

anonymous · Answer

yeah, got it...tanx alot

anonymous · Answer

Welcome :)

anonymous · Answer

please what the answer to this question

anonymous · Answer

is it a, b, c,or d which

anonymous · Answer

whats the answer to this equation please