Submarine Fires Torpedo, Submarine's Recoil?

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In summary: I'll email my professor about it, thanks for your help though!In summary, the conversation discusses a problem involving a submarine firing a torpedo and the calculation of the initial recoil speed of the submarine. The correct calculation is done using conservation of momentum, but the program administering the online assignment does not accept the correct answer of -0.01. Attempts with different significant figures and variations also do not produce the accepted answer.
  • #1
Schoomy
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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 isn't 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?
 
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  • #2
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
Schoomy said:

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 isn't 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
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
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
so for m1v1 (torpedo) is the initial mass the sub plus the torpedo?

Not sure I follow, can you be more specific?
 
  • #7
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
zero...
 
  • #9
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
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
Do you happen to know what the correct answer is? How do you know it's not the accepted answer?
 
  • #12
Our assignments are administered online via webassign.com.

It says -0.01 is wrong...
 
  • #13
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
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
Schoomy said:
Our assignments are administered online via webassign.com.

It says -0.01 is wrong...

And do they say + .01 is incorrect?
 
  • #17
Yes,

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

Very odd
 

1. How does a submarine fire a torpedo?

A submarine fires a torpedo by using a torpedo tube, which is a long cylindrical tube that extends from the submarine's hull. The torpedo is loaded into the tube and then ejected by pressurized air or water. The torpedo is then propelled by its own engine.

2. What causes a submarine's recoil when firing a torpedo?

A submarine's recoil when firing a torpedo is caused by the force of the torpedo being ejected from the tube. The sudden release of pressurized air or water creates a force that pushes the submarine in the opposite direction.

3. How is the recoil from a submarine's torpedo managed?

The recoil from a submarine's torpedo is managed by the submarine's propulsion system. The engines are designed to counteract the force of the recoil, allowing the submarine to maintain its position and stability.

4. Can a submarine fire multiple torpedoes at once?

Yes, a submarine can fire multiple torpedoes at once. Depending on the size and type of the submarine, it can have multiple torpedo tubes that can fire simultaneously. This allows for a higher chance of hitting a target or defending against multiple threats.

5. Are there any safety concerns when firing a torpedo from a submarine?

Yes, there are safety concerns when firing a torpedo from a submarine. One of the main concerns is the potential for the torpedo to malfunction or malfunction during launch, which could cause damage to the submarine. Additionally, proper safety protocols must be followed to ensure the safety of the crew and the surrounding environment.

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