When you rub a balloon on a sweater, for example, some electrons come off and end up on the balloon. The fibers have lost electrons giving them a positive charge. The rubber gained electrons giving it a negative charge. ... The positively charged fibers are now attracted to the negatively charged balloon.
I think it's matter or density
In a force diagram set-up, we name the angle of inclination theta, g as the acceleration due to gravity. In this case, the forces acting on the box going down is the weight itself impeded by the friction between the box and the inclined plane.
The weight of the box is expressed as mg sin theta
The frictional force is expressed as the normal force times the coefficient of friction that is expressed as mu g cos theta.
By Newton's second law of motion, F = ma = mg sin theta - mu g cos theta
Thus, a = g (sin theta - u cos theta