Torque: Types, Calculation, Measurement

What is Torque?

Torque is a measurement of the force required to rotate an object around an axis. In linear kinematics, force causes an object to accelerate; in angular kinematics, torque causes an object to accelerate.

The understanding of Torque is innate. People are aware of its existence but might struggle to put it in words. However, it can easily be explained by picturing a door or a window. To open a door, you push the door away from the hinge. This allows the door to be opened with very little force. The closer you get to the hinge, the more force is required. Although the work done is the same in both circumstances (the larger force is applied over a smaller distance), people prefer to use less force, which is why the door handle is usually located all the way towards the opposite end as far away from the hinge as possible.

Before we go any further, let us examine the types of torque

Types of Torque:

1. Static.
2. Dynamic.

The term “static torque” refers to a torque which doesn’t create an angular acceleration. Because the door does not rotate on its hinges despite the force applied, someone pushing on it applies a static torque to it. Because they are not accelerating, someone pedaling a bicycle at a steady speed is likewise applying a static torque.

A dynamic torque is carried by the drive shaft of a racing car accelerating from the start line because it must produce an angular acceleration of the wheels provided that the car is accelerating along the course.

This might appear confusing but what should be understood is that torque basically measures the increase in the angular movement of the object that the force is being applied to. When a car is accelerating, it carries dynamic torque, whereas, when the car is moving at a steady speed, a static torque is being applied by the driveshaft that maintains the speed. Likewise, when a car is decelerating, a dynamic torque is applied by the brakes.

How is torque calculated?

The symbol of Torque is τ. Torque is a vector force. The angle is extremely important when it comes to torque. 

The magnitude of the Torque vector, τ, for a torque produced by a given force, F, is 

Τ = F⋅rsin(θ)

Where r is the length of the moment arm and θ  is the angle between the force vector and the moment arm.

If the force is at right angle to the moment arm, for example, while opening a door, the sine becomes 1 and the formula then becomes

Τ = F⋅r

Measurement of Torque

The SI unit for torque is the Newton-meter. 

In a non-rotating system, measuring a static torque is usually simple and involves measuring a force. The torque can be calculated immediately from the length of the moment arm. Torque measurement in a rotating system is much more complicated. One way involves monitoring strain in the metal of a torque-delivering drive shaft and transmitting the data wirelessly.

What is rotational equilibrium?

For a rotational system, the concept of rotational equilibrium is identical to Newton’s first law. Unless acted on by an external torque, an object that is not rotating stays that way. Similarly, unless acted on by an external torque, an object rotating at constant angular velocity will continue to rotate.

When dealing with difficulties involving several torques acting on a rotating item, the idea of rotational equilibrium comes in handy. In this scenario, it’s the net torque that matters. If a rotatable object’s net torque is zero, it will be in rotational equilibrium and unable to gain angular acceleration.

What is the difference between Torque, Energy and Power?

Torque, power, and energy are all terms that are often used interchangeably. For example, an engine’s torque is sometimes wrongly referred to as its ‘turning power.’

The dimensions of torque and energy are the same (i.e., they may be represented in the same fundamental units), yet they are not the same thing. Torque is a vector quantity that can only be specified for a rotatable system.

If the rotational speed is known, however, power can be computed from torque. In fact, an engine’s horsepower is usually derived from measured torque and rotational speed rather than being measured directly.

Day to day examples of situations where torque is observed:

• Using a screwdriver to turn a screw
• Accelerating and decelerating in a car
• Swinging a yo-yo like a pendulum