Answer
B. F=ma
Explanation
The Newton's laws of motion tries to explain the how bodies behave and the energy changes when theys are in motion. For the 3 of them to hold, the bodies must be moving in a straight line and with constant velocity.
The second one states that, "the change of momentum of a moving body is directly proportional to the force producing it and it takes place to the direction of force."
From the choices given, the appropriate answer is B. F=ma
Answer:
Equilibrium. • When an object is in equilibrium (either at rest or moving with constant velocity), the net force acting on it zero.
Answer
Given,
Length of the wire,L = 31 cm = 0.31 m
mass of the wire, m = 9 g = 0.009 Kg
Current in the wire,I = 7 A
Magnetic field strength, B= ?
Equating magnetic force to the weight of the wire.



B = 0.0407 T
For Force to be upward magnetic field direction should be outward of the plane of paper.