Percussion (physique)

In classical mechanics, impulse (symbolized by J or Imp) is the change in momentum of an object. If the initial momentum of an object is p1, and a subsequent momentum is p2, the object has received an impulse J: Momentum is a vector quantity, so impulse is also a vector quantity. Newton’s second law of motion states that the rate of change of momentum of an object is equal to the resultant force F acting on the object: so the impulse J delivered by a steady force F acting for time Δt is: The impulse delivered by a varying force is the integral of the force F with respect to time: The SI unit of impulse is the newton second (N⋅s), and the dimensionally equivalent unit of momentum is the kilogram metre per second (kg⋅m/s). The corresponding English engineering unit is the pound-second (lbf⋅s), and in the British Gravitational System, the unit is the slug-foot per second (slug⋅ft/s). Impulse J produced from time t1 to t2 is defined to be where F is the resultant force applied from t1 to t2. From Newton's second law, force is related to momentum p by Therefore, where Δp is the change in linear momentum from time t1 to t2. This is often called the impulse-momentum theorem (analogous to the work-energy theorem). As a result, an impulse may also be regarded as the change in momentum of an object to which a resultant force is applied. The impulse may be expressed in a simpler form when the mass is constant: where F is the resultant force applied, t1 and t2 are times when the impulse begins and ends, respectively, m is the mass of the object, v2 is the final velocity of the object at the end of the time interval, and v1 is the initial velocity of the object when the time interval begins. Impulse has the same units and dimensions (MLT−1) as momentum. In the International System of Units, these are kg⋅m/s = N⋅s. In English engineering units, they are slug⋅ft/s = lbf⋅s. The term "impulse" is also used to refer to a fast-acting force or impact. This type of impulse is often idealized so that the change in momentum produced by the force happens with no change in time.