Momentum is mass times velocity, and it never appears or disappears on its own — it only changes hands. Watch two identical balls collide head-on: one stops dead, the other moves off at exactly the same speed.
Momentum is a way of measuring "how much motion" an object carries: it is mass times velocity. A heavy object moving fast carries a lot of momentum; a still object carries none. Because velocity has a direction, so does momentum — the two balls here move along one straight track, so we can simply add and subtract their amounts.
The diagram shows two identical balls on a frictionless track. Ball A rolls in from the left at speed v; Ball B sits still. When they meet, they push on each other for a brief instant, and that push passes momentum from A to B. For equal masses in a straight, elastic hit the result is beautifully simple: A stops and B leaves at speed v.
During the contact the two balls exert equal and opposite forces on each other, so whatever momentum one loses the other gains. Add them up and the total before equals the total after — momentum is conserved. Nothing was manufactured; the same quantity just changed hands.
Ball A carries momentum m × v to the right. Ball B is at rest, so its momentum is zero. The total momentum of the pair is therefore just m × v.
At the moment of contact the balls press on each other. According to Newton's third law those forces are equal and opposite, and they last for the same brief time in both directions.
Because the forces are equal and opposite, the momentum one ball loses is exactly the momentum the other gains. No momentum is created or destroyed — it merely moves from A to B.
With equal masses and a head-on, elastic hit, all of A's momentum goes to B. That leaves A stationary and sends B away to the right at the very same speed A arrived with.
Check the sums: before, m × v + m × 0; after, m × 0 + m × v. The two are equal, which is exactly what conservation of momentum says.
Momentum is mass times velocity: a heavy object moving fast carries a lot of it, and a still object carries none. It is a vector, so its direction matters as much as its size.
Total momentum is conserved in every collision, elastic or not, as long as no outside force acts on the objects. Kinetic energy, however, is only conserved in a perfectly elastic collision.
For a head-on hit between equal masses, the maths of conservation gives a simple result: the incoming ball stops and the other leaves with the incoming speed. The momentum has simply changed hands.
An elastic collision conserves both momentum and kinetic energy. A near-perfect example is two hard steel balls or the balls of a Newton's cradle; most real collisions lose some energy as heat and sound.
Usually yes, because momentum is mass times velocity. At the same speed the far heavier truck carries much more momentum, which is why it needs a much longer distance to stop.