Foundational Physics
Short Answer: In the ideal rope model, a rope can pull when it is taut, but it cannot push. A rope is flexible: if you try to push along it, it bends or goes slack rather than transmitting a steady compressive force. The force a taut rope exerts on an attached object is called tension, and it points along the rope, away from the object.
Sensei’s Warning
A rope may be drawn as a straight line in a free-body diagram, but that does not make it a rigid rod. Never add a “pushing tension” force to make an ideal rope hold an object in place.
What happens when you pull on a rope?
Picture a box attached to a rope. Grab the other end and pull. The rope becomes taut, and the box feels a force toward your hand.
In a free-body diagram, that force is tension. It acts along the rope and points away from the box, toward the rope.
What happens if you try to push the box with the rope?
Now move your hand toward the box without touching the box. Instead of pushing the box, the rope folds, sags, or bunches up. A flexible rope cannot remain straight under an ordinary compressive load.
In the ideal rope model, we describe that situation as a slack rope. The rope's tension is then zero. This is why an ideal rope can pull but cannot push.
What if the rope is already tight?
A tight rope can pull on both objects attached to its ends. The rope pulls each object toward the rope's interior; the directions at the two ends are opposite.
That is still pulling, not pushing. The rope remains under tension rather than compression.
How is a rigid rod different?
A rigid rod can transmit a push because it can resist compression. For example, you can push a box with a straight stick, but not with a loose string.
The difference is the model of the connection, not a special rule about the direction you want the box to move.
The takeaway
A taut ideal rope pulls along its length; a slack rope exerts no tension. It cannot push. When you draw a free-body diagram, make the tension arrow point away from the attached object and along the rope.
When This Breaks
Real cables and ropes may have bending stiffness, weight, or contact forces that the ideal flexible, massless-rope model ignores. A specially stiff cable or a rope in contact with other objects can behave differently from the simplified classroom model.
For ordinary introductory-physics problems, however, an ideal rope transmits tension only. A compressive force belongs to a different idealization, such as a rigid rod.
Keep Exploring: Newton's Laws and Free-Body Diagrams
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