Balanced forces

Two forces act on the book: gravity = 12 N downward and normal reaction force = 12 N upward. These forces act in opposite directions.

When forces act in opposite directions, subtract them: $\text{Resultant}=12\text{N (up)}-12\text{N (down)}=0\text{N}$

The resultant force is 0 N, meaning the forces are perfectly balanced. The book remains stationary because balanced forces cause no change in motion.

We need to find the resultant force — the overall force when all forces are combined, taking direction into account.

These answers would only be correct if just one force was acting. Since two equal and opposite forces act, they cancel each other — neither direction wins. Both options are incorrect.

'24 N upward' would be correct if both forces acted in the same direction (adding them together). But gravity acts downward and the normal reaction acts upward — opposite directions, so they subtract, not add.

Because $12\text{N}-12\text{N}=0\text{N}$, the resultant force is 0 N. The forces are balanced.

A non-contact force acts between objects without any physical contact. Examples include gravity and magnetism.

Gravity acts on the ball even when nothing is touching it — the Earth pulls the ball downward through empty space. No contact is needed.

Gravity pulling a ball toward the ground is a non-contact force because the Earth exerts a gravitational pull on the ball without physically touching it.

We need to find the option where the force acts without physical contact between objects.

Friction requires two surfaces to be in contact — the box and the floor must touch for friction to act. This is a contact force.

A hand pushing a door requires direct physical contact between the hand and the door. This is clearly a contact force.

Brakes work by pressing brake pads against the wheel — this is friction, a contact force requiring physical touching.

Only gravity pulling a ball toward the ground acts without any physical contact, making it the non-contact force.