Submarine Fires Torpedo, Submarine's Recoil?

  • Thread starter Schoomy
  • Start date
  • #1
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Homework Statement



A submarine of mass 2.5 x 10^6 kg and initially at rest fires a torpedo of mass 260 kg. The torpedo has an initial speed of 100.4 m/s. What is the initial recoil speed of the submarine? Neglect the drag force of the water.

Homework Equations



m1v1=-m2v2

The Attempt at a Solution



mass one = 2.5x10^6 (submarine)
velocity one = 0 (sub isnt moving)
mass two = 260 (torpedo)
velocity two = 100.4 (torpedo)

How can I find the recoil? I thought maybe it'd just be velocity one (instead of zero solve for x)

v1 = (260*100.4)/2.5x10^6 = -0.01 m/s
But that's wrong...

what am i missing?
 

Answers and Replies

  • #2
51
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I think you have the right idea, but you need to do it like this:

m1v1 (of sub) + m1v1 (of torpedo) = m2v2 (of sub) + m2v2 (of torpedo)
and since initial velocity of the sub is 0

m1v1 (of torpedo) = m2v2 (of sub) + m2v2 (of torpedo)

I think this is right, hope this helps!
 
  • #3
LowlyPion
Homework Helper
3,090
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Homework Statement



A submarine of mass 2.5 x 10^6 kg and initially at rest fires a torpedo of mass 260 kg. The torpedo has an initial speed of 100.4 m/s. What is the initial recoil speed of the submarine? Neglect the drag force of the water.

Homework Equations



m1v1=-m2v2

The Attempt at a Solution



mass one = 2.5x10^6 (submarine)
velocity one = 0 (sub isnt moving)
mass two = 260 (torpedo)
velocity two = 100.4 (torpedo)

How can I find the recoil? I thought maybe it'd just be velocity one (instead of zero solve for x)

v1 = (260*100.4)/2.5x10^6 = -0.01 m/s
But that's wrong...

what am i missing?
They are asking for speed and not velocity. So I would expect they are not looking for a negative number.
 
  • #4
42
0
Yeah, but if we have:


m1v1 (of torpedo) = m2v2 (of sub) + m2v2 (of torpedo)

doesnt m1v1 (torpedo) = m2v2 (torpedo) ? thus you'd just subtract one from the other, giving zero, resulting in:

0 = m2v2 (of sub)

Result would be undefined. What am I missing?
 
  • #5
159
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Initially in the problem, before the torpedo is fired, what is the momentum of the system? The total momentum after the torpedo is fired must be the same.
 
  • #6
42
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so for m1v1 (torpedo) is the initial mass the sub plus the torpedo?

Not sure I follow, can you be more specific?
 
  • #7
159
0
Yes it would be. However, I think you're missing the point. The sub initially has no velocity, and the torpedo initially has no velocity. Therefore, what is the initial momentum of the system?
 
  • #8
42
0
zero...
 
  • #9
159
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That's correct. If the initial momentum of the system is zero, then because of the conservation of momentum, the final momentum of the system must be zero. Does that help you at all?
 
  • #10
42
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I still don't get it...

Initial Momentum of System (aka zero) = m_sub*v_sub + m_torpedo*v_torpedo

Rearrange:

- m_sub*v_sub = m_torpedo*v_torpedo

Resulting in:

- (2500000)(recoil) = (260)(100.4)

-Recoil = ((260)(100.4))/2500000 = -0.0104416, which isn't accepted answer...
 
  • #11
159
0
Do you happen to know what the correct answer is? How do you know it's not the accepted answer?
 
  • #12
42
0
Our assignments are administered online via webassign.com.

It says -0.01 is wrong...
 
  • #13
159
0
Have you tried it without the negative sign? And/or with more or less significant figures? I don't know why that answer would be wrong. However, if you're required to have the correct number of significant figures based on the problem, your answer should be 0.010.

I hope one of these solutions works for you
 
  • #14
42
0
Odd...

The program allows for a 2% error and I've tried with any number of significant figures/variations (this is just practice, so I can try as many times as I want with no penalty)

Just frustrating because I understand all other recoil problems except this one...
 
  • #16
LowlyPion
Homework Helper
3,090
4
Our assignments are administered online via webassign.com.

It says -0.01 is wrong...
And do they say + .01 is incorrect?
 
  • #17
42
0
Yes,

These are wrong:
-0.01
0.01
0.010, etc
0.01044etcetc

Very odd
 

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