percobaan

Single particle

The momentum of a particle is traditionally represented by the letter p. It is the product of two quantities, the mass (represented by the letter m) and velocity (v):^[1]

The units of momentum are the product of the units of mass and velocity. In SI units, if the mass is in kilograms and the velocity in meters per second, then the momentum is in kilograms meters/second (kg m/s). Being a vector, momentum has magnitude and direction. For example, a model airplane of 1 kg, traveling due north at 1 m/s in straight and level flight, has a momentum of 1 kg m/s due north measured from the ground.

Many particles

The momentum of a system of particles is the sum of their momenta. If two particles have masses m₁ and m₂, and velocities v₁ and v₂, the total momentum is

The momenta of more than two particles can be added in the same way.
A system of particles has a center of mass, a point determined by the weighted sum of their positions:

If all the particles are moving, the center of mass will generally be moving as well. If the center of mass is moving at velocity v_cm, the momentum is:

This is known as Euler's first law.^[2][3]

Relation to force

If a force F is applied to a particle for a time interval Δt, the momentum of the particle changes by an amount

In differential form, this gives Newton's second law: the rate of change of the momentum of a particle is equal to the force F acting on it:^[1]

If the force depends on time, the change in momentum (or impulse) between times t₁ and t₂ is

The second law only applies to a particle that does not exchange matter with its surroundings,^[4] and so it is equivalent to write

so the force is equal to mass times acceleration.^[1]
Example: a model airplane of 1 kg accelerates from rest to a velocity of 1 m/s due north in 1 s. The thrust required to produce this acceleration is 1 newton. The change in momentum is 1 kg m/s.