Final answer:
To replace three forces with an equivalent force-couple system at point B, calculate the resultant force by vector addition and consider the moments created by forces not acting through point B. The resultant force and moment together form the equivalent system.
Step-by-step explanation:
The question is asking about how to replace three forces with an equivalent force-couple system at a point B. This involves summing the forces acting on an object and finding their resultant force and the couple, which involves moments that create rotation but no translation. In this scenario, the student should first calculate the resultant of the three given forces A, B, and C by vector addition, which in this case involves adding the magnitudes considering their directions (to the right for A, to the left for B, and downward for C). To ensure the object moves straight down, the horizontal components (forces to the right and left) must cancel each other out. The equivalent force system will then consist of the resultant force and a couple resulting from the forces not acting through point B, creating rotation around point B.
In terms of a free-body diagram and using the concept of force per unit length (F/l), this would involve combining the forces into a single resultant force and a moment at point B. For an object placed between two contact surfaces, as A and B, the true force can be found by considering both the normal forces and the friction forces acting on the object. The couple (moment) would be the product of the force and the perpendicular distance from point B to the line of action of the force. When all forces and moments (couples) are accounted for, the force-couple system at B will be the equivalent representation of the original forces acting on the body.