Physics Help Forum Momentum of Rocket

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Feb 16th 2018, 11:46 PM #1 Junior Member   Join Date: Apr 2017 Location: Turkey Posts: 22 Momentum of Rocket Greetings to all I have two small questions about the Momentum of a Rocket and conservation of Momentum , the questios are : 1- Can the speed of a Rocket exceeds the exhaust speed of the Fuel ? 2- A bomb, initially at rest, explodes into several pieces. (a) Is linear momentum conserved? (b) Is kinetic energy conserved? Explain In the attachement Photo , questions number 20 and 22 Thanks in Advance Best regards Razi Attached Thumbnails
 Feb 17th 2018, 01:37 AM #2 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 987 Your list of questions are a good way of applying the basic concepts to a variety of situations. Have you no thoughts on these? For instance you have only asked about the second part of question 20, what was your answer to the first part? Do you have a mathematical formula for momentum and kinetic energy?
Feb 18th 2018, 05:18 AM   #3
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 Originally Posted by Razi Greetings to all I have two small questions about the Momentum of a Rocket and conservation of Momentum , the questios are : 1- Can the speed of a Rocket exceeds the exhaust speed of the Fuel ? 2- A bomb, initially at rest, explodes into several pieces. (a) Is linear momentum conserved? (b) Is kinetic energy conserved? Explain In the attachement Photo , questions number 20 and 22 Thanks in Advance Best regards Razi
We're required by forum rules not to simply give you the answers to your question but to help lead you to the answer. Can you take a shot at answering these questions yourself, please?.

I'll give you a two hints:

(1) Try thinking in terms of absolutes by which I mean is it possible for one object to be expelled (exhaust) by another object (rocket) in which the speed of the first object is less than the second object? What would happen if the first object was a bowling ball and the second object a flea?

(2) Conservation of a quantity means that the total before equals the total after.

(a) What is the total linear momentum before the explosion and what is it after the explosion? Find the answer by using the laws of conservation of linear momentum.

(b) What is the total kinetic energy before the explosion and what is it after the explosion? How do you find the answer?

If that doesn't lead you to your answer then please let me know. In such case turn to your text on the subject of conservation of momentum and kinetic energy first.

Feb 20th 2018, 02:10 AM   #5
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 about the second question which was ∶ a bomb initially at rest ,explodes into several pieces .(a)is linear momentum conserved ?(b)is kinetic energy conserved ? explane ? My answer is∶ the momentum is not conserved ?but the kinetic energy is conserved because the kinetic energy of all picese qual to the chemical potential enegry which was into the bomb before exploding . ( i am not sure about this answer )
Let's get momentum cleared up first.

Momentum is conserved.

Initially the bomb is at rest so has zero momentum in any direction
Momentum is a vector quantity.
Hence direction is also important.

I see from your formulae that you understand momentum can be positive or negative.

So the momentum equation for the explosion event is:

Vector sum of momentum after explosion = Vector sum before explosion = 0 in this case.

You will find that the vector sum, taking signs into account, of the moementa of all the fragments adds up to zero.

Of course we add the components in three mutually perpendicular directions, where they will separately add to zero in each of those directions.

Now for the energy.

The question asks specifically for the kinetic energy.

Again initially the bomb is at rest so the kinetic energy is initially zero.

So yes the total of chemical and kinetic energies (and any others that may be involved) is constant ie preserved.

But the kinetic energy is one half mass (always positive) times the velocity squared (again always positive).

KE is always positive.

So the kinetic energies of all the fragments are all positive (unlike the momenta)

And they are moving so their sum is not zero.

So kinetic energy is not conserved

This is why it is so useful to have both momentum and KE available for calculation.

Would you like to revisit your other answers in the light of this?

 Feb 27th 2018, 03:40 AM #6 Junior Member   Join Date: Apr 2017 Location: Turkey Posts: 22 Dear Studiot Thank you so much for your explanation and really you have explained it clearly , now this problem is very clear to me , thanks again and please accept my best regards . Razi

 Tags momentum, rocket

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