The weight is 40N, the dynamic friction coefficient is 0.25, and the specific sliding friction force is 10N. That is, if you give it a force greater than or equal to 10N, it will move. When the force is less than 10N, the friction force is equal to and opposite to the force you give to the object, which belongs to static friction force. When it is greater than 10N, the friction force is 10N, and the friction force is constant with the increase of the acting force. At this time, the friction force is dynamic. Extra force is used to change the speed of an object.