Tensile force generally refers to the force exerted on an object, such as a rope, chain, or cable, when it is subjected to an external force that attempts to stretch or tighten it by attempting to separate the molecules that hold it together.
This tension force acts in the opposite direction to the force applied to keep the object in equilibrium.
The magnitude of the tension force depends on the material and geometry of the object, as well as the external force applied. In the case of an ideally inextensible rope or cable, the tension force at any point along its length is the same.
If, for example, you have a horizontal rope held at both ends and you hang an object in the middle, the tension force in the rope will be equal at both ends and at the point where the object is hung.
Formula of tension
In the case of an ideally inextensible rope, the tension force (T) can be calculated using the following formula:
T = F / cos(θ)
Where:
- T = Tension force in the string.
- F = External force applied to the rope.
- θ = Angle between the rope and the direction of the external force (angle of inclination of the rope).
This formula is valid for ideal situations, where the rope is inextensible and there are no additional considerations such as friction or elasticity of the material.
In more complex situations, determining the tensile strength may require more detailed analysis and consideration of other factors and laws such as Hooke's law for elastic materials.
Examples of tension forces
Tension forces are found in numerous contexts in our daily lives. Here are some examples where you can observe these types of forces:
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Ropes and Cables: When you pull on a rope or cable, whether to lift an object, close a door, or tow a vehicle, you are applying a tension force to the rope or cable.
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Suspension bridge: Suspension bridges use cables to support their deck. The cables are subjected to tension forces that act to support the weight of the deck and the vehicles crossing the bridge.
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Steel-reinforced concrete is an ingenious combination that takes advantage of steel's ability to resist tensile stresses to compensate for concrete's low tensile strength. In contrast, concrete has excellent compressive strength.
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Elevator: In an elevator, the cable that holds the cabin and allows up and down movement is subjected to tension forces to support the weight of the passengers and the cabin itself.
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Power Transmission Lines: Overhead power lines that carry electricity from power plants to homes and businesses are made up of cables subjected to tension forces to support the weight of the cable and electrical conductors.
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Kites: When you fly a kite, the string holding it up is under tension, allowing the kite to stay in the air.
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Parachutes: Parachutes used for the controlled descent of people or equipment also rely on tension in their cords to deploy and remain stable during descent.
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Boat moorings: Docked boats use ropes or cables called mooring lines to hold them to the dock, and these mooring lines are subject to tensile forces due to waves and currents.
Use and applications of tension forces
Tension force is present in our daily lives and in various fields of physics and engineering.
Let's look at some of its most important uses:
- Stability and balance: In structures such as suspension bridges, tension in the cables allows the deck to remain suspended and support the weight of vehicles crossing. Without this force, the bridge would not be able to stand.
- Lifting and pulling: Tension is key to lifting objects, such as in elevators. When we press the button to raise, the rope tightens and allows for lifting. It is also used in vehicle towing, where the rope connects a car in trouble to another vehicle.
- Safety in engineering: In the design of buildings and bridges, it is crucial that materials can withstand the expected tensile forces. This prevents structural failures and ensures the safety of people.
- Sports and Recreation: In activities such as climbing or archery, tension in ropes and bows is essential to perform these activities safely and effectively.
- Transmission of forces: In pulley systems, the tension in the ropes allows heavy objects to be moved more easily.
- Electricity and communication: In power lines, tension is necessary to hold wires in place and ensure effective transmission of electricity and data.
- Scientific experiments: In research, stress is measured to evaluate the properties of materials and their strength.
Measuring tensile strength
Tension force is measured directly by the application of suitable measuring devices or instruments that can capture the magnitude of the force exerted on a rope, cable or material that is under tension.
There are different types of devices and techniques for measuring tensile strength, and the choice depends on the context and type of object or material being evaluated. Some common methods for measuring tensile strength include:
- Dynamometers : Dynamometers are portable devices that measure tension and compression forces. They can be used to measure the tension force in ropes, cables, or elements subject to tension.
- Load cells : These are sensors that measure the force applied in a specific direction. They are used in a wide variety of applications, such as weighing objects, measuring force in industrial machines, or stress in structures.
- Extensometers : These devices measure the deformation of a material under stress and convert it into a measure of the applied force. They are commonly used in materials testing to evaluate their mechanical properties.
- Testing Machines : To evaluate the mechanical properties of materials, testing machines are used that apply controlled forces and measure the response of the material under stress.
- Load Testing : In construction and civil engineering, load testing is performed on structures to measure their ability to withstand tension and compression forces.
Difference with the surface tension force
This concept should not be confused with that of surface tension.
Surface tension refers to the force acting on the surface of a liquid that tends to reduce its surface area to the minimum possible. It is a physical property that is due to the cohesion forces between the molecules of the liquid.
At a free surface of the liquid, such as at the interface between liquid and air, the molecules inside the liquid are attracted inward due to cohesive forces, resulting in a kind of "skin" on the surface.
Surface tension is responsible for some interesting phenomena, such as the formation of spherical drops and bubbles, since a spherical shape minimizes the surface area and therefore minimizes the surface energy of the liquid.