Answer:
a change of position in relation to another object.
Explanation:
Motion can be defined as a change in the location (position) of a physical object or body with respect to a reference point.
This ultimately implies that, motion would occur as a result of a change in location (position) of an object with respect to a reference point or frame of reference i.e where it was standing before the effect of an external force.
A reference point refers to a location or physical object from which the motion (movement) of another physical object or body can be determined.
Mathematically, the motion of an object is described in terms of acceleration, time, distance, speed, velocity, displacement, position, etc.
Hence, motion is a change of position in relation to another object.
is the mass at time zero, while 
is the half-life of the element.
, t=121.0 s and 
, so we can find the initial mass 