#kinetic energy

56 messages · Page 1 of 1 (latest)

slim grotto
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A rocket of mass M when empty carries a mass M of fuel. The rocket and fuel travel at speed v.
The engine of the rocket is fired and all of the fuel is expelled. The speed of the rocket increases
to 2v.
What happens to the kinetic energy of the rocket?
A It doubles.
B It halves.
C It increases by a factor of four.
D It stays the same.

wet capeBOT
late thistle
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!status

wet capeBOT
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What step are you on?
1. I don't know where to begin.
2. I have begun but got stuck midway.
3. I got an answer but I was told that it's wrong.
4. I got an answer and would like my work checked.
5. I have a question about someone else's work/solution.
6. I have completed the problem and don't need help anymore. Thank you.
7. None of the above
slim grotto
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4

late thistle
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!show

wet capeBOT
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Show your work, and if possible, explain where you are stuck.

slim grotto
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So as M+M js 2M

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And kinetic energy is mv^2 /2

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So we get mv^2

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But when fuel is exhausted and velocity doubles

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So

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M(2v)^2 /2

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We get 2mv^2

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And it doubles

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But the marking scheme says otherwise

late thistle
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it looks good to me
but the marking scheme said it's 4 times?

slim grotto
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Yes

late thistle
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the flaw is that

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i mean the tricky part

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is that

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"what happens to the KE of the rocket

slim grotto
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Initial is 1/2 mv^2?

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And then 1/2 :2

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Is 1:4

late thistle
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without saying anything about the fuel

slim grotto
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I did this but teacher said u need to take fuel in equation

late thistle
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i see

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let's do it again then

slim grotto
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Marking scheme js certified like it js never wrong

slim grotto
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So

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What i thought

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I initially was

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That fuel mass is just for confusion

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As a trick

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We only take rocket mass as M

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And velocity as V

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So ee get 1/2 Mv^2

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Then we do mass as M/2 and velocity as 2v

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So we get 2mv^2

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And 1/2 : 2 is 4

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So this is the approach?

late thistle
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thinking

slim grotto
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In online sources they all also did factor of four

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Bro be writing thesis

late thistle
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Initial
1/2 M v² (KE of rocket) + 1/2 M v² (KE of fuel)
Final
1/2 M (2v²) (KE of rocket) + ?(KE of fuel)

slim grotto
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We just take rocket

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They asked for rocket only

late thistle
slim grotto
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As 0 mass

late thistle
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to be fair, this is math server, im not that good at physics

slim grotto
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Thank You for the time

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Allah Hafiz

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.close