Why does the dynamometer show at the same time the modulus of sliding friction?

Because the bar moves uniformly, then the resultant of the friction force and the elastic force acting on it is zero. This means that the sliding friction force is equal in modulus and oppositely directed to the elastic force.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.