An object's momentum is the product of its velocity and mass. The quantity describes, for instance, the impact that a moving vehicle has on an object that it hits or the penetrative power of a speeding bullet. When the object travels at a constant speed, it neither gains nor loses momentum. When two objects collide, they again together gain and lose no momentum. The only way for a body to gain momentum is for an external force to act on it.
Divide the magnitude of the external force on the object by the object's mass. For this example, imagine a force of 1,000 Newtons acting on a mass of 20 kg: 1,000 ÷ 20 = 50. This is the object's acceleration, measured in meters per second squared.
Multiply the acceleration by the time for which the force acts. If the force acts, for instance, for 5 seconds: 50 × 5 = 250. This is the object's change in velocity, measured in m/s.
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Multiply the object's change in velocity by its mass: 250 × 20 = 5,000. This is the object's change in momentum, measured in kg m/s.