Momentum in Classical mechanics (dt: “Impuls”)
If an object is moving in any reference frame, then it has momentum in that frame. The amount of
momentum that an object has depends on two variables: the mass and the velocity of the moving
object in the frame of reference. This can be written as:
momentum = mass × velocity
In physics, the symbol for momentum is a small p, so the above equation can be rewritten as:
where m is the mass and v the velocity. The SI unit of momentum is kilogram metres per
second (kg* m/s).
For example: What is the momentum of a 5-kg bowling ball moving at 2 m/s?
Which one is greater - the momentum of a girl (m=50kg) walking at 1m/s or of a bird (m=1kg)
flying at 4m/s?
Conservation of momentum and collisions (dt: “Impulserhaltung und Stöße”)
Momentum has the special property that it is always conserved, even in collisions. Kinetic energy,
on the other hand, is not conserved in collisions if they are inelastic. Since momentum is conserved
it can be used to calculate unknown velocities following a collision.
A common problem in physics that requires the use of this fact is the collision of two particles.
Since momentum is always conserved, the sum of the momentum before the collision must equal
the sum of the momentum after the collision:
where the subscript i signifies initial, before the collision, and f signifies final, after the
There are two basic kinds of collisions, both of which conserve momentum:
Elastic collisions conserve kinetic energy
Inelastic collisions don't conserve kinetic energy
A collision between two pool or snooker balls is a good example of an almost totally elastic
collision. In addition to momentum being conserved when the two balls collide, the sum of kinetic
energy before a collision must equal the sum of kinetic energy after:
A common example of a perfectly inelastic collision is when two objects collide and then stick