An elastic collision<span> is where there is no loss of kinetic energy in the </span>collision<span>. Momentum is conserved saved in </span>inelastic collisions<span>, but cannot track the kinetic energy through the </span>collision<span> since some of it is changed into other forms of energy.</span>
For the same reason that you can skate around a curve at constant speed but not with constant velocity.
The DIRECTION you're going is part of your velocity, but it's not part of your speed.
If the DIRECTION changes, that's a change of velocity.
The object doesn't have to change speed to have a different velocity. A change of direction is enough to do it.
And any change of velocity is called acceleration.
Answer:
Rest and motion are the relative terms because they depend on the observer's frame of reference. So if two different observers are not at rest with respect to each other, then they too get different results when they observe the motion or rest of a body.
