When the net force acting on a body is not equal to zero, then the body will be in a state of motion in the direction of the greater magnitude of force. Hence, diagrams B and D indicate motion. Hence, option D is correct.
Calculating the net force in each scenario :
Diagram A :
Fgrav - Fnorm = 3 - 3 = 0
Ffric - Fapp = 5 - 5 = 0
Net force = 0 (No motion)
Diagram B :
Fgrav - Fnorm = 3 - 3 = 0
Ffric - Fapp = 5 - 0 = 5
Net force = 5N (in the direction of the frictional force)
Diagram C:
Fgrav - Fnorm = 20 - 20 = 0
Ffric - Fapp = 0
Net force = 0 (No motion)
Diagram D:
Fair - Fgrav = 40 - 25 = 15N
Ffric - Fapp = 0
Net force = 15 N (in the upward direction)
Therefore, the diagrams B and D exhibits motion.
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