Everything Science Grade 12

6.4 CHAPTER 6. MOTIONIN TWO DIMENSIONS

Inelastic Collisions ESCDE

DEFINITION: Inelastic Collisions

An inelastic collision isa collision in which total momentum is con-

served but total kinetic energy is not conserved. The kineticenergy is

transformed into other kinds of energy.

So the total momentumbefore an inelastic collisions is the same as afterthe collision.
But the total kinetic energy before and afterthe inelastic collision is different. Of course
this does not mean that total energy has not beenconserved, rather the energy has been
transformed into another type of energy.

As a rule of thumb, inelastic collisions happen when the colliding objects are distorted
in some way. Usually they change their shape. The modification of the shape of an
object requires energy and this is where the “missing” kinetic energy goes. A classic
example of an inelasticcollision is a motor caraccident. The cars change shape and
there is a noticeable change in the kinetic energy of the cars before andafter the colli-
sion. This energy was used to bend the metal and deform the cars. Another example
of an inelastic collisionis shown in Figure 6.7.

pim, KEim pia, KEia

Before collision

pf, KEf

After collision

Figure 6.7: Asteroid moving towards the Moon.

An asteroid is moving through space towards the Moon. Before the asteroid crashes
into the Moon, the totalmomentum of the systemis:

pi= pim+ pia

The total kinetic energyof the system is:

KEi= KEim+ KEia

When the asteroid collides inelastically with the Moon, its kinetic energy is transformed
mostly into heat energy. If this heat energy is large enough, it can cause the asteroid
and the area of the Moon’s surface that it hits, tomelt into liquid rock! From the force
of impact of the asteroid, the molten rock flows outwards to form a crateron the Moon.

After the collision, the total momentum of the system will be the sameas before. But
since this collision is inelastic, (and you can see thata change in the shape of objects
has taken place!), total kinetic energy is not the same as before thecollision.
Momentum is conserved:
pi= pf