In this case to find the weight of an object you must use the formula.
W = mg
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This is possible due to inertia of motion. which is nothing but newton's first law.
according to this law , an object tries to retain its state of motion or rest unless acted upon by an external force.
consider an object placed on a paper, initially both the object and paper are at rest. to pull the paper , we apply force on the paper and paper gains velocity. but the object keeps its motion of rest and hence the paper can be removed without moving the object.
Answer:
Option (A)
Explanation:
Displacement of a particle on a velocity time graph is represented by the area between the line representing velocity and x-axis (time).
Displacement of a particle from t = 0 o t = 40 seconds = Area of ΔAOB
Area of triangle AOB = 
= 
= 80 m
Similarly, displacement of the particle from t = 40 to t = 80 seconds = Area of ΔBCD
Area of ΔBCD = 
= 80 m
Total displacement of the particle from t = 0 to t = 80 seconds,
= 80 + 80
= 160 m
Option (A) will be the answer.