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Old September 25th, 2009, 05:14 PM
TGS TGS is offline
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Default How to calculate the final velocity.

A bullet of mass 0.205 kg traveling horizontally at a speed of 250 m/s embeds itself in a block of mass 3.5 kg that is sitting at rest on a nearly frictionless surface.

What is the speed of the block after the bullet embeds itself in the block?
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Old September 25th, 2009, 05:26 PM
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A bullet of mass 0.205 kg traveling horizontally at a speed of 250 m/s embeds itself in a block of mass 3.5 kg that is sitting at rest on a nearly frictionless surface.

What is the speed of the block after the bullet embeds itself in the block?
Use conservation of momentum.
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Old September 25th, 2009, 05:57 PM
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I've already tried that... I got it wrong.
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Old September 25th, 2009, 06:12 PM
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I've already tried that... I got it wrong.
Please show your work and I will review it.
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Old September 25th, 2009, 06:41 PM
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m_1 v_{1i} + m_2 v_{2i} = m_1 v_{1f} + m_2 v_{2f}

m_1 v_{1i} + 0 = 0 + m_2 v_{2f}

(0.205 kg) (250m/s) = (3.5kg) (v_{2f})

v_{2f} = 14.643

Last edited by mr fantastic; September 25th, 2009 at 06:44 PM. Reason: Improved the latex and formatting
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Old September 25th, 2009, 06:47 PM
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Originally Posted by TGS View Post
m_1 v_{1i} + m_2 v_{2i} = m_1 v_{1f} + m_2 v_{2f}

m_1 v_{1i} + 0 = 0 + m_2 v_{2f}

(0.205 kg) (250m/s) = (3.5kg) (v_{2f}) Mr F says: Your mistake is here.

v_{2f} = 14.643
Since the bullet is embedded in the block, the correct equation is (0.205 kg) (250) = (3.5 + 0.205) (v_{2f}) ....
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Old September 25th, 2009, 06:54 PM
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ohhhhh... thank you.
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