What is momentum?
Mass times velocity, measured in kilogram metres per second. It captures how hard something is to stop: a lorry at 20 mph and a car at 60 mph can carry similar momentum despite very different speeds. Because velocity is a vector, momentum has direction too — two objects of equal mass moving in opposite directions at the same speed have equal and opposite momentum, summing to zero.
What does conservation of momentum mean?
In any closed system with no external forces, total momentum before an interaction equals total momentum after. It holds in every collision, explosion and recoil, whether or not energy is conserved. It is why a rifle recoils, why a rocket accelerates by expelling gas, and why two skaters pushing apart move in opposite directions with momenta that cancel exactly.
What is the difference between elastic and inelastic collisions?
Momentum is conserved in both; kinetic energy only in elastic ones. In a perfectly elastic collision — billiard balls come close — objects bounce apart and total kinetic energy is unchanged. In an inelastic collision energy is lost to deformation, heat and sound. In a perfectly inelastic collision the objects stick together and move as one, which loses the most energy while still conserving momentum exactly.
How does momentum differ from kinetic energy?
Momentum is mass times velocity; kinetic energy is half mass times velocity squared. That squaring matters: doubling speed doubles momentum but quadruples kinetic energy. It is why impact damage rises so steeply with speed, and why a crash at 60 mph is four times as energetic as one at 30, not twice. Momentum tells you about stopping; energy tells you about damage.
What is impulse and how does it relate?
Impulse is force times the time it acts, and it equals the change in momentum. Rearranged, force equals change in momentum divided by time — so extending the duration of a stop reduces the force. Airbags, crash barriers, catching a ball with moving hands and gymnasts rolling on landing all exploit this. The momentum change is fixed; the force is not.
Can momentum be negative?
Yes, and the sign is meaningful rather than a quirk. Momentum is a vector, so once you choose a positive direction, anything moving the other way has negative momentum. Getting the signs right is essential in collision problems — treating both objects as positive is the most common source of wrong answers, because it makes a head-on collision look like a chase.