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# Final momentum

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## Richard Datwyler

on 28 May 2014

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#### Transcript of Final momentum

Final Momentum
Two last examples
Explosions
Rockets
Explosions are examples of inelastic collisions run backwards.

The energy won't be conserved, ever.
Usually they start out at rest, but they don't have to.
If they are at rest, you will have 0 = final momentum
otherwise you will have some initial to deal with.
Because they are inelastic, you can only use conservation of momentum, which means there can only be 2 unknowns.
let us start with a firework of mass 30 kg moving east at a speed of 9 m/s.
It explodes into 3 pieces, with masses 10, 15 and M.
If the 10 kg mass flies 30 degrees to the north of east with a speed of 17 m/s
the 15 kg flies 45 degrees to the south of east with a speed of 13 m/s
Find the mass M and its speed and direction?
Yet, because mass is conserved you can add one more unknown, but it is straight forward.
m = 10 kg
v = 17 m/s
a = 30
0
0
m = 15 kg
v = 13 m/s
a = 45
m=?
v=?
a=?
m = 30 kg
v = 9 m/s
a = 0
Rockets
Rockets change our conservation of momentum, not only does the velocity change, but the mass can too. This happens as they expel the fuel.
Let us have a 25,000 kg rocket that expels its fuel at a constant rate of 3500 kg/s at a constant velocity of 150 m/s.
What is the initial acceleration of this rocket?
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