What happens when two forces act in the opposite directions on an object?

Correct — D. Force is a vector, so two forces on the same line combine with sign, not by blind addition.

Same direction: the magnitudes add.
Opposite directions: the magnitudes subtract, and the net force is the difference between the two forces, pointing the way the larger force points.

Push a box right with 10 N while friction drags it left with 4 N and the box feels 6 N to the right. Make the two equal and the difference is zero: the forces are balanced and the motion does not change.

Why the others are wrong

• (a)
  The net force acting on the object is the total of the two forces. — Adding is the same-direction rule. Apply it here and a tug-of-war between two equal teams would produce an enormous force instead of none.
• (b)
  The net force acting on the object is the difference between the two objects. — 'Objects' is not a quantity you can subtract. The paper prints 'the difference between the two objects', and a difference between objects means nothing in mechanics — what subtracts is the two forces. This is the near-miss the item is built on.
• (c)
  The net force acting on the object is the sum of the two objects. — Wrong operation and wrong noun. Summing belongs to forces acting the same way, and it is forces that combine, not the objects.

A force has a size and a direction, so combining forces is vector addition rather than ordinary arithmetic.

Along a single line the rule collapses to something simple: same direction, add the magnitudes; opposite directions, subtract them. The leftover force acts in the direction of the bigger one.

When the two are equal and opposite the net force is zero. The forces are then balanced, and by Newton's first law the object carries on doing what it was already doing — sitting still, or moving steadily.

Key facts

• Forces acting in the same direction add, and forces acting in opposite directions subtract.
• The net force of two opposite forces acts in the direction of the larger force.
• Equal and opposite forces give a net force of zero, so the state of motion does not change.
• Force is measured in newtons (N).

Common traps

• Picking the option that says 'difference' without noticing it says difference between the two objects.
• Forgetting that the leftover force still has a direction, that of the larger force.

No directly related past PYQ was found.