That's what happens if there is more than one force acting on the
object, and the forces are balanced, that is, they all "cancel out".
Think of the rope in a Tug-'o-War. It has 50 musclebound football guys
all pulling the rope to the west, and 150 strong cheerleaders all pulling
the rope to the east. The total force to the west is exactly equal to the
total force to the east, and the rope doesn't move at all. The forces on it
are balanced, and the effect on its motion is the same as if there were
no force on it at all.
Answer:
External force on him will be 112 N
Explanation:
We have given the mass of the sprinter m =70 kg
Acceleration of the sprinter 
We have to find the net external force
According to second law of motion force = mass ×acceleration
Force is dependent on the mass and acceleration
So 
So external force will be 112 N
The answer to this question is a) sulfur
