Momentum Conservation, Recoil, and Explosions
If external impulse during a short interaction is negligible, total linear momentum of the chosen system is conserved even when kinetic energy changes or internal energy is released.
Why this shows up in the exam
Explosions into fragments · Gun and shell recoil · People or blocks pushing apart
Learn the idea
Internal forces redistribute momentum while total isolated momentum stays fixed. An explosion or recoil can create large opposite velocities, but the vector sum of all fragment momenta still equals the pre-event momentum.
🧠 Memory hook: Choose the system before conserving its momentum.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- sum p_i = sum p_f — isolated-system momentum conservation
- P_system = M V_cm — total momentum and center-of-mass velocity
- Delta K = K_f-K_i — energy change may be nonzero
How to approach it
- 1Define the isolated system
- 2Resolve initial momentum into components
- 3Set component totals equal after the event
Common slip-ups that cost marks
- •Conserving kinetic energy in an explosion
- •Dropping vector directions
- •Including an external body inconsistently
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A 2 kg body speeds up from 3 m/s to 7 m/s. What net work is done on it?
More from Work, Energy and Power
Work and its calculation
Work is the energy transferred by a force acting over a distance, and can be calculated using the dot product, area under a force-displacement graph, or for variable and constant forces.
Conservation of energy
The law of conservation of energy states that energy cannot be created or destroyed, only transformed, including cases with energy loss and efficiency considerations.
Work-energy theorem
The work-energy theorem states that the net work done on an object equals the change in its kinetic energy, and applies to both constant and variable forces.
Conservative and non-conservative forces
Conservative forces, like gravity and spring force, conserve mechanical energy, while non-conservative forces, like friction, dissipate energy as heat.
Elastic potential energy
Elastic potential energy is the energy stored in a stretched or compressed spring, proportional to the square of its displacement.
Power
Power is the rate at which work is done or energy is transferred, and can be calculated as the product of force and velocity at any instant.