Linear momentum is a vector quantity defined as the product of an object's mass, m, and its velocity, v. Linear momentum is denoted by the letter p and is called “momentum” for short: Note that a body'smomentum is always in the same direction as its velocity vector. The units of momentum are kg.
 Linear momentum is a measure of an object's translational motion. The linear momentum p of an object is defined as the product of the object's mass m times its velocity v. 

p = mv. 

Linear momentum is a vector. Its direction is the direction of the velocity. The Cartesian components of p are 

px = mvx, py = mvy, pz = mvz. 

If an object's velocity is changing, its linear momentum is changing. We have 

dp/dt = d(mv)/dt. 

If the mass of the object is constant then 

dp/dt = mdv/dt = ma. 

We write 

dp/dt = F. 

This is a more general statement of Newton's second law which also holds for objects whose mass is not constant. 

If an object receives an impulse, its momentum changes. We may write 

dp = Fdt. 


If the force acting on the object is constant, then 

∆p = F∆t. 

The integral of force over time is called the impulse I of the force. We have shown that the impulse I is equal to the change in momentum ∆p. You give an object an impulse, by letting a force act on it for a time interval ∆t. 

I = ∆p = Favg∆t 

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