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How to find the energy of an object that was at rest?

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1. Homework Statement
In two rockets, one of which moves and the other is at rest, the motors are connected for a short time. During their operation they throw the same mass of gas (small in comparison with the mass of the rocket) at the same speed with respect to the rockets. The kinetic energy of the rocket in motion was ##E_0##, increase by 4 percent. Determine the energy of the second rocket

2. Homework Equations
##E_k=\frac{1}{2}mv^2##

3. The Attempt at a Solution
I first wrote the final kinetic energy of the second rocket: ##E_k=E_0+\frac{4}{100}E_0##, then by using that equation: ##\frac{1}{2}mv_f^2=\frac{1}{2}mv_0^2+\frac{1}{25}(\frac{1}{2}mv_0^2)=mv_f^2=mv_0^2+\frac{1}{25}mv_0^2\Rightarrow m_vf^2=\frac{25mv_0^2+mv_0^2}{25}=\frac{26mv_0^2}{25}\Rightarrow 25v_f^2=26v_0^2##. After that I don't know what to do.
 

haruspex

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1. Homework Statement
In two rockets, one of which moves and the other is at rest, the motors are connected for a short time. During their operation they throw the same mass of gas (small in comparison with the mass of the rocket) at the same speed with respect to the rockets. The kinetic energy of the rocket in motion was ##E_0##, increase by 4 percent. Determine the energy of the second rocket

2. Homework Equations
##E_k=\frac{1}{2}mv^2##

3. The Attempt at a Solution
I first wrote the final kinetic energy of the second rocket: ##E_k=E_0+\frac{4}{100}E_0##, then by using that equation: ##\frac{1}{2}mv_f^2=\frac{1}{2}mv_0^2+\frac{1}{25}(\frac{1}{2}mv_0^2)=mv_f^2=mv_0^2+\frac{1}{25}mv_0^2\Rightarrow m_vf^2=\frac{25mv_0^2+mv_0^2}{25}=\frac{26mv_0^2}{25}\Rightarrow 25v_f^2=26v_0^2##. After that I don't know what to do.
What relates the change in speed of the rocket to the mass and relative speed of the exhaust?
 
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What relates the change in speed of the rocket to the mass and relative speed of the exhaust?
I don't know.
 

haruspex

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That momentum increases?
Well, yes, ejectng the exhaust gases increases the momentum of the rocket - that's how rockets work.
But by how much is the momentum of the rocket increased if a mass M of gas is exhausted at speed v relative to the rocket?
 
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Well, yes, ejectng the exhaust gases increases the momentum of the rocket - that's how rockets work.
But by how much is the momentum of the rocket increased if a mass M of gas is exhausted at speed v relative to the rocket?
It is increasing by 4%, so it would be ##P_f=P_i+\frac{1}{25}P_i##, right?
 

jbriggs444

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The kinetic energy of the moving rocket has increased by 4%. That does not mean that its momentum has increased by 4%.

You had already correctly calculated that:
##25v_f^2=26v_0^2##
That should allow you to compute by how much the velocity of the moving rocket had increased.
 
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The kinetic energy of the moving rocket has increased by 4%. That does not mean that its momentum has increased by 4%.

You had already correctly calculated that:

That should allow you to compute by how much the velocity of the moving rocket had increased.
From that I would have ##v_f=\sqrt{\frac{26}{25}v_0\Rightarrow v_f=1.01v_0##, am I right?
 

haruspex

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From that I would have ##v_f=\sqrt{\frac{26}{25}}v_0\Rightarrow v_f=1.01v_0##, am I right?
Yes, except that 1.01 is rather inaccurate. Anyway, don't bother about the arithmetic for now. What does momentum conservation allow you to deduce?
 
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Yes, except that 1.01 is rather inaccurate. Anyway, don't bother about the arithmetic for now. What does momentum conservation allow you to deduce?
Well, since the momentum of the system is constant, so the sum of the both rockets momentum must be constant, right?
 

haruspex

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Well, since the momentum of the system is constant, so the sum of the both rockets momentum must be constant, right?
The two rockets are in different systems. The untethered rocket, together with its exhaust gases, is an isolated system.
 
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The two rockets are in different systems. The untethered rocket, together with its exhaust gases, is an isolated system.
Then the momentum of the gas should be the same as the momentum increase.
 

haruspex

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Then the momentum of the gas should be the same as the momentum increase.
Right, and what do we know about the momentum of the gas from the other rocket?
 
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Right, and what do we know about the momentum of the gas from the other rocket?
Well, since the rocket was at rest, the momentum of the system must be zero, so after the exhaust, the momentum of the gas must be the same as the momentum of.tje rocket but negative.
 

haruspex

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Well, since the rocket was at rest, the momentum of the system must be zero, so after the exhaust, the momentum of the gas must be the same as the momentum of.tje rocket but negative.
Yes, but can you connect this with the momentum change of the first rocket?
 
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Yes, but can you connect this with the momentum change of the first rocket?
Yes. Since both rockets connect their engines at the same time and thy exhaust the same mass of gas, then the change of momentum of the first rocket must be the same as the change of the rocket at rest, right?
 

haruspex

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Yes. Since both rockets connect their engines at the same time and thy exhaust the same mass of gas, then the change of momentum of the first rocket must be the same as the change of the rocket at rest, right?
Yes.
 
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So, to calculate the momentum of the rocket at rest I just need to use the change in the velocity of the second rocket, which was ##1.01v_i##? ##P=mv_f=m(1.01v_i)##, and so the kinetic energy from the rocket at rest would be ##E_k=\frac{1}{2}m(1.01v_i)^2##. But why did you said that it is rather inaccurate?
 

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