In physics, tension is a force developed in a string, rope, or cable when it is stretched due to a force. Tension is exerted along the length of the wire/ cable, but in a direction, opposite to the force applied to it. Tension can also be referred to as stress, tautness, or tensity. Let us look at the formula, and examples of the tension force in our daily lives.
Tension force is a type of contact force, as this can only be exerted by contact between the cable and the object. This force basically pulls but never pushes. Pushing the cable reduces the tension, whereas pulling it increases the tension.
Tension is equal and opposite for an ideal case of rope with zero mass. Suppose a weight of 50 N is hanging from a rope, then the tension exerted on the rope will be 50 N in the upward direction. The tension balances out the weight of the object. Tension is opposite to compression forces.
What is the advantage of tension force?
Tension allows us to transfer force across large distances. Suppose we want to pull a bucket that is deep down in the well, and we cannot go down there physically. The tension in the rope allows us to transfer our force through the rope to lift the object out.
Unit of tension force
Tension is a type of force. So, the SI unit of tension will be Newton. It can also be expressed in other units of force like dyne, poundal, etc. You can read our blog on units of force for more information.
Examples of tension force
Image by Military_Material from Pixabay
- Dogs pulling a sled. The force of the dogs can be transferred to the sled efficiently.
- The tension of the rope can be used to draw water from wells through pulleys.
- Cables from helicopters are used for rescue missions, lifting people, or heavy equipment.
- The tension of the cables is used for rock climbing or scaling rough terrain.
- Elevators use cables that can withstand high-tension forces.
- A network of cables is used to hold onto large bridges. Such structures are called suspension bridges. The Golden Gate Bridge in San Francisco is an example of a suspension bridge.
- Tension is developed in the rope during a game of tug of war.
The formula of tension force
There is no specific formula for tension. Tension is always equal and opposite to the force applied to it(ideal case).
Consider this example of rock climbing. The tension of the rope is equal to the force on it by the person. The force exerted by the person is equal to the weight of the person. Hence tension, in this case, will be the weight = m * g in the upward direction.
Image by billcosmos from Pixabay
Let’s look at another example. Pulleys are used to lift heavy objects. The tension in the rope transfers our force applied in the downward or horizontal direction to lift the object in the upward direction.
Let us now at an example of the cables involved in the movement of an elevator. The value of tension in the cables is different in different states of motion. This is due to the fact that it depends on whether the elevator is moving against gravity or moving in the direction of gravity.
- When the elevator is at rest, the tension in the cable is equal to m*g.
- When the elevator is moving up against gravity, the tension is equal to m(g+a)
- When the elevator is moving down with gravity, the tension is equal to m(g-a)
The value of the maximum tension of the cable limits the weight that it can carry. That is why the number of persons allowed in an elevator is limited.
Is the value of tension positive or negative?
Tension is always positive. If the value of tension is negative then that means that the string is under compression. The value will be positive for any force that pulls the rope. And, the value will be negative for any compressive force.
