Couple

Students can define couple in Physics within a few lines of explanation. When two equal and parallel forces act opposite to each other, then they both create a couple.

There's no ambiguity in understanding the couple definition Physics. A couple has the only effect of producing or preventing the turning effect of the body.

A couple can be calculated with the help of the product of the magnitude of both forces & the perpendicular distance between the forces’ line. These force lines are also called action lines.  

Couple Moments Physics

When you try to understand and define couple, the steering wheel of an automobile can be the best option. It would help if you had hand forces to create a couple. Also, the application of a screwdriver twisted by your hand is responsible for the creation of a couple.

(Image will be Uploaded Soon) 

A couple in Physics is a twisting force that does not allow any kind of moment, turning effect throughout the entire process of force engagement. 

This is a different type of force that comes with the application of two other forces. These two forces are responsible for the twisting of an object. A couple is dependent on certain other methods such as several forces, their directions, methods, their nature.

Application of Couple in Physics

You can know different types of applications for couples. Some of them are given hereunder:

• Steering wheel applied by the car driver

• Opening and closing of a water tap

• Winding the spring of an alarm clock

• Unlocking the locker by using a key 

• Opening and closing of a cap of a water bottle, or jug.

• Turning of a screwdriver

Difference Between Couple and Moment

Students can understand the actual difference that lies within a couple and moment with the help of the given table:

Couple Force Formula

As we know, a couple is the combination of two parallel forces that have identical magnitude but opposite directions distinguished by a perpendicular distance.

M\[_{c}\] = r * F

The above formula is using vector analysis. As both of the forces are acting at a distance, they create a moment. This is called a moment of a couple.

We can name a couple as torque also. It is responsible for the development of the rotational motion of a body. Forces that are involved in a couple have the same magnitude but different directions. 

This is why the resultant force is zero. When you don’t find any resultant force acting on the body, the body won’t possess any type of translational motion.

When there is no translational motion, then the body is at rest. According to research, the moment of the couple in the same plane about any point is not equal to zero. As a result, we obtain that the body is under rotational motion. 

Couple vs Torque

Torque and work have identical dimensions. However, they are not the same if we consider their physical quantities. The moment of force is a vector quantity, whereas work is a scalar quantity. 

It is coincident that both of them possess identical dimensions, but their purposes are completely different. The measurement of the couple is not linked with the axis of rotation. That is why couple and torque both are independent of the axis of rotation.

However, the magnitude of a couple is constant. When a body is under translational equilibrium, the resultant force acting on that body is zero.

Mathematically, we can write it as

ΣF = 0

This expression is ideal for an equilibrium body.

Also, when a body comes under the influence of rotational equilibrium, then the resultant moment acting on a body should be zero. The moment also has the same impact when it has no rotational motion:

ΣM = 0

Force Couple Examples

When the forces caused due to two hands help to turn a steering wheel are considered the best example of a couple. 

Each hand grips of a driver on the wheel at different points have an impact over the shaft. When a driver applies a force, then an equal amount of magnitude and opposite direction helps to rotate the wheel. The movement of screwdrivers is also another example of a couple.

What is the definition of a couple?

A screwdriver is twisted by the equivalent of a couple of hand forces, whereas an automobile's steering wheel is spun by a pair of hand forces. In contrast, a wrench is operated by a force exerted at only one end, resulting in an imbalanced force on the element being tightened in addition to the turning moment.

A couple is a pair of equal parallel forces pointing in opposite directions in mechanics. The only thing a couple can do is cause or prevent a body from turning. The amount of either force is multiplied by the perpendicular distance between their action lines to compute the turning effect, or moment, of a pair.

The Couples' Characteristics

• The couple does not induce translational motion because the two forces that make up the couple are equal and opposing.

• When it is applied to a body, the net resultant force is zero.

• Because the algebraic sum of the moments of the two forces around any point in their plane is not zero, it causes pure rotational motion in the body.

• The size and direction of a couple's moment about any point on its plane are both constant.

The Couple's Moment

The moment of the pair is described mathematically as the product of the force and the perpendicular distance between the two forces' lines of action. The arm of the Couple refers to the perpendicular distance between the lines of action of two forces. That is, the product of the applied force and the arm of a pair of forces equals the moment of force.

As a result, the moment of a pair of forces is equal to,

Τ = F × D

(Image will be uploaded soon)

The moment of a pair of forces will be bigger if the formula for the moment of a pair of forces is followed.

• The force's magnitude is bigger, and

• The arm of the pair of forces is longer, indicating that the perpendicular distance between the two forces' lines of action is greater.

The Newton-Meter is the SI unit for Couple Moment (N m). The dimensional formula for the Moment of Couple is \[\left [ ML^{2}T^{-2} \right ]\].

Couple Moments of Various Types

A couple of moments can be divided into two categories:

Positive moment: When the body rotates in an anticlockwise direction under the impact of a Couple, the moment is said to be positive.

Negative moment: The moment is considered to be negative when the force pair spins in a clockwise direction.