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(9.8) To evaluate the integral, we need to know how the force varies with time. The quantity impulse of the force F acting on a parti- cle over the time interval (t ! t f " t i . Impulse is a vector defined by (9.9) Equation 9.8 is an important statement known as the impulse–momentum theorem: 3 I ! % t f t i
F
dt ( p ! p f " p i ! % t f t i
Fdt S E C T I O N 9 . 2 • Impulse and Momentum 257 The impulse of the force F acting on a particle equals the change in the momen- tum of the particle. This statement is equivalent to Newton’s second law. From this definition, we see that im- f " t i . The direc- tion of the impulse vector is the same as the direction of the change in momentum. Im- Because the force imparting an impulse can generally vary in time, it is convenient to define a time-averaged force (9.10) where (t ! t f " t i . (This is an application of the mean value theorem of calculus.) Therefore, we can express Equation 9.9 as (9.11) I ! F
( t F ! 1 ( t
% t f t i
F
dt t
i t
f t
i F (a) t
f t F (b) t F Area = F ∆t Figure 9.4 (a) A force acting on a particle may vary in time. The im- pulse imparted to the particle by the force is the area under the force-versus-time curve. (b) In the time interval (t, the time-averaged force (horizontal dashed line) gives the same impulse to a particle as does the time-varying force de- scribed in part (a). Airbags in automobiles have saved countless lives in acci- dents. The airbag increases the time interval during which the passenger is brought to rest, thereby decreasing the force on (and resultant injury to) the passenger. Courtesy of Saab 3 Although we assumed that only a single force acts on the particle, the impulse–momentum theo- rem is valid when several forces act; in this case, we replace F in Equation 9.8 with "F. Impulse of a force Impulse–momentum theorem |