|
|
S E C T I O N 9 . 7 • Rocket Propulsion 277 Conceptual Example 9.17 Exploding Projectile A projectile fired into the air suddenly explodes into several Solution Neglecting air resistance, the only external force Figure 9.26 (Conceptual Example 9.17) When a projectile explodes into several fragments, the center of mass of the system made up of all the fragments follows the same parabolic path the projectile would have taken had there been no explosion. Example 9.18 The Exploding Rocket After the explosion, where v f is the unknown velocity of the third fragment. Equating these two expressions (because p i ! p f ) gives v f ! What does the sum of the momentum vectors for all the ("
240 iˆ # 450jˆ) m/s ! M(300 jˆ m/s) M 3
v f # M 3 (240 iˆ m/s) # M 3 (450 jˆ m/s) p f ! M 3 (240 iˆ m/s) # M 3 (450 jˆ m/s)# M 3
v f A rocket is fired vertically upward. At the instant it reaches Solution Let us call the total mass of the rocket M; hence, p i of the rocket just be- fore the explosion must equal the total momentum p f of the fragments right after the explosion. Before the explosion, p i ! M v i ! M(300
jˆ m/s) location of the center of mass of the system (bear plus b x b ! m p x p . (Unfortunately, you cannot return to your spiked shoes and so you are in big trouble if the bear shown, noting your location. Take off your spiked shoes, p , and how far the bear slides, x b . The point where you meet the bear is the fixed 9.7 Rocket Propulsion When ordinary vehicles such as cars and locomotives are propelled, the driving force The operation of a rocket depends upon the law of conservation of linear momentum as applied to a system of particles, where the |