Which mechanical device can only transmit a tensile force along its length?

Which mechanical device can only transmit a tensile force along its length?

• A. Equilibrium equation in component form
• B. Free body Diagram
• C. String or Cable
• D. Linear Spring

Answer: (C)

The important idea here is that some elements can only carry load in one mode along their length: tension. A string or cable does exactly that—it pulls on the objects it connects, with the force directed along its length, but it cannot push. If a compressive tendency acts on a string, it goes slack and stops transmitting force, so the connection only ever conveys a tensile force. This is why a string or cable is the right choice: it embodies tension-only transmission along its length. In contrast, a free-body diagram is just a visualization tool for forces acting on a body, not a device that transmits force. Equilibrium equations are mathematical relationships used to analyze those forces, not transmitters themselves. A linear spring resists axial deformation and can carry both tension and compression depending on the load, so it isn’t limited to transmitting only tensile forces.

The important idea here is that some elements can only carry load in one mode along their length: tension. A string or cable does exactly that—it pulls on the objects it connects, with the force directed along its length, but it cannot push. If a compressive tendency acts on a string, it goes slack and stops transmitting force, so the connection only ever conveys a tensile force.

This is why a string or cable is the right choice: it embodies tension-only transmission along its length. In contrast, a free-body diagram is just a visualization tool for forces acting on a body, not a device that transmits force. Equilibrium equations are mathematical relationships used to analyze those forces, not transmitters themselves. A linear spring resists axial deformation and can carry both tension and compression depending on the load, so it isn’t limited to transmitting only tensile forces.