Submarine Fires Torpedo, Submarine's Recoil?

[Schoomy]

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?

 

[aimslin22]

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!

 

[LowlyPion]

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.

 

[Schoomy]

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?

 

[Mattowander]

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.

 

[Schoomy]

so for m1v1 (torpedo) is the initial mass the sub plus the torpedo?

Not sure I follow, can you be more specific?

 

[Mattowander]

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?

 

[Schoomy]

zero...

 

[Mattowander]

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?

 

[Schoomy]

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...

 

[Mattowander]

Do you happen to know what the correct answer is? How do you know it's not the accepted answer?

 

[Schoomy]

Our assignments are administered online via webassign.com.

It says -0.01 is wrong...

 

[Mattowander]

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

 

[Schoomy]

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...

 

[Schoomy]

I have a similar problem (somewhat different concept...vertical height acheived).

https://www.physicsforums.com/showthread.php?p=2154921&posted=1#post2154921

We can't seem to figure it out...care to take a look for me?

 

[LowlyPion]

Our assignments are administered online via webassign.com.

It says -0.01 is wrong...

And do they say + .01 is incorrect?

 

[Schoomy]

Yes,

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

Very odd