To fully determine the effect of a force acting on a rigid body, which properties must be known?

Introduction / Context:
When analyzing forces in statics and dynamics, it is not enough to know how big a force is. The full effect on a rigid body depends on where and in what direction the force acts. This question tests understanding of the complete specification of a force system.

Given Data / Assumptions:

• Rigid body assumptions (no deformation).
• Force represented as a vector applied along a line of action.
• Sense of action (push or pull) matters for boundary conditions and contact behavior.

Concept / Approach:
A force applied to a rigid body is fully described by: (1) magnitude, (2) direction and line of action (including point or line of application), and (3) sense (push vs. pull). Changing the line of action, even with the same magnitude and direction, can change the induced moment and thus the body’s response.

Step-by-Step Solution:

Verification / Alternative check:
Consider two equal forces of 100 N parallel to each other but displaced by a distance: together they can form a couple even if magnitudes are equal, proving line of action matters.

Why Other Options Are Wrong:

• Only magnitude: Insufficient because moments depend on position and direction.
• Only line of action: Without magnitude, the effect cannot be quantified.
• Only nature (push/pull): Does not determine size or rotational effects alone.

Common Pitfalls:
Ignoring point of application in beam problems; treating all forces of the same magnitude as equivalent regardless of their lines of action.

Final Answer:
All of the above