In order to slide a huge box lying on the ground in her room, Reshma should…

Static friction is the resistive force that acts between two surfaces in contact whenever there is no relative sliding between them. As a push on a stationary object increases, static friction increases to match it, up to a maximum (limiting) value; the object begins to slide only once the applied force exceeds this maximum static friction.

For the box lying on the floor of Reshma's room, this means the horizontal push she applies must be greater than the static friction between the box and the floor before the box starts moving.

Why the other forces do not set this threshold:

• Gravitational force acts vertically downward on the box because of its weight; it has no horizontal component, so it does not itself resist the horizontal sliding, though it does help determine how strongly the floor pushes back on the box (the normal force).

• Normal force is the floor's vertical reaction to the box's weight, acting perpendicular to the floor; like gravity, it does not act along the direction of sliding.

• Muscular force is simply the force Reshma herself generates and applies to the box through her muscles — it is the acting force in the situation, not a separate resisting force to be overcome.

By Newton's second law, only forces acting along the direction of intended motion — here, horizontal — can resist or cause that motion. Among the four options, static friction is the only horizontal resisting force on the box, confirming that Reshma's applied force must exceed static friction for the box to slide.