Chapter
7
Momentum
and Collisions
Collisions
Part 3 and Rockets
Elastic
Collisions
Example 722
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.
Solution
Example
723
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?
Solution
Rocket
Propulsion
The
force exerted on the rocket by the ejected fuel is referred to
at the thrust.
Thrust
Units: N 
Increase in
Speed

Example 724
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?
Solution
Example 725
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.
Solution
(a)
(b)
(c)
On
to the Practice Multiple Choice 