A 20-kg box sits on a rough horizontal surface. When a horizontal force of $120 \mathrm{~N}$ is applied, the object accelerates at $1.0 \mathrm{~m} / \mathrm{s}^{2} .$ (a) If the applied force is doubled, the acceleration will (1) increase, but less than double; (2) also double;
(3) increase, but more than double. Why? (b) Calculate the acceleration to prove your answer to part (a).