Final answer:
To increase the force of friction on the washing machine without changing the surfaces, you can increase the normal force applied to the washing machine.
Step-by-step explanation:
Friction is a force that resists the motion between two surfaces in contact. To increase the force of friction on the washing machine without changing the surfaces, you can increase the normal force applied to the washing machine. The normal force is the force exerted by a surface that is perpendicular to the surface. In this case, placing additional weight on top of the washing machine would increase the normal force and consequently increase the force of friction. For example, you can place a heavy object on top of the washing machine, or if possible, fill the washing machine with more clothes.