Chapter 7

Momentum and Collisions

Collisions Part 3 and Rockets

Elastic Collisions

Example 7-22

At an amusement park, a 96.0 kg bumper car moving with a speed of 1.24 m/s bounces elastically off a 135 kg bumper car at rest. Find the final velocities of the cars.



Example 7-23

A hover fly is happily maintaining a fixed position about 10 ft above the ground when an elephant charges our of the bush and collides with it. The fly bounces elastically off the forehead of the elephant. If the initial speed of the elephant is , is the speed of the fly after the collision equal to (a) , (b) 1.5, or (c) 2?



Rocket Propulsion

The force exerted on the rocket by the ejected fuel is referred to at the thrust.


Units: N


Increase in Speed


Example 7-24

The ascent stage of the lunar lander was designed to produce 15,500 N of thrust at liftoff. If the speed of the ejected fuel is 2500 m/s, what is the rate at which the fuel must be burned?



Example 7-25

A rocket whose initial mass is 850 kg consumes fuel at the rate = 2.3 kg/s. The speed v of the exhaust gases relative to the rocket engine is 2800 m/s. (a) What thrust does the rocket engine provide? (b) What is the initial acceleration of the rocket? (c) Suppose, instead, that the rocket is launched from a spacecraft already in deep space, where we can neglect any gravitational force acting on it. The mass of the rocket when its fuel is exhausted is 180 kg. What is its speed relative to the spacecraft at the time? Assume that the spacecraft is so massive that the launch does not alter its speed.






On to the Practice Multiple Choice