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Explosions and fragmentation
Intermediate
Total momentum before and after the explosion remains the same.
An explosion converts internal energy into kinetic energy while total momentum stays unchanged. The fragments' momenta sum to the original momentum.
A 6 kg shell at rest breaks into 2 kg and 4 kg pieces; the 2 kg piece at 6 m/s means the 4 kg moves 3 m/s oppositely.
Kinetic energy increases, momentum doesn't change.
In more detail
In an explosion, the fragments fly apart with total momentum equal to the momentum before the explosion, which is often zero. The centre of mass keeps its original path if no external forces act.
Real-world uses
- Fireworks shells break into fragments whose total momentum is conserved.
- Engineers analyse controlled demolition and fragmentation of rock.
- A firework that explodes into pieces sends the fragments flying in directions that add up to zero total momentum.
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Frequently asked questions
If the total momentum is zero, does nothing move?
No. The fragments move in opposite directions so their momenta cancel.
Is kinetic energy conserved in an explosion?
No. Chemical energy is released and converted into kinetic energy of the fragments.
Why do the fragments move in different directions?
Their momenta are shared between them, so the vector sum of all fragment momenta equals the original momentum of zero.
Related topics in Momentum & Collisions
Centre of mass · Velocity of centre of mass · Centre-of-mass frame · Variable mass: rocket equation · Thrust · Multi-stage rockets