Definition of Couple in Physics on Vedantu
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.
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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
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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.
FAQs on Couple
1. What is a couple in Physics?
In physics, a couple is defined as a pair of forces that are equal in magnitude and parallel to each other but act in opposite directions. The primary effect of a couple is to produce a turning or rotational effect on a body without causing any linear (translational) motion.
2. What is the formula to calculate the moment of a couple?
The moment of a couple, which is a measure of its turning effect, is calculated by multiplying the magnitude of one of the forces (F) by the perpendicular distance (d) between their lines of action. The formula is: Moment (τ) = F × d. The SI unit for the moment of a couple is the Newton-meter (N·m).
3. Can you give some real-world examples of a couple?
Several everyday actions involve the application of a couple. Common examples include:
- Turning a car's steering wheel with both hands.
- Opening or closing a water tap.
- Unlocking a door with a key.
- Twisting the cap off a bottle or jar.
- Using a screwdriver to turn a screw.
4. What are the main properties of a couple?
A couple has several distinct properties that set it apart from a single force:
- The net force of a couple is zero, which is why it does not produce any translational motion.
- It produces a pure rotational motion, meaning its only effect is to make an object turn.
- The moment of a couple is constant and its effect is the same about any point in its plane of action.
5. Why does a couple only produce rotation and not translation?
A couple produces only rotation because it consists of two forces that are equal in magnitude and opposite in direction. When these forces are added as vectors, their sum (the resultant force) is zero. According to Newton's first law, an object's translational motion only changes if there is a net external force. Since the net force is zero, there is no linear acceleration, and thus no change in translational motion. However, because the forces act along different lines, they create a net turning effect, or torque, causing rotation.
6. How is the moment of a couple different from torque?
The term 'torque' is often used interchangeably with the 'moment of a couple,' as both measure a turning effect. However, there is a subtle distinction. Torque is a general term for any rotational force, which can be produced by a single force acting at a distance from an axis. A couple specifically refers to a system of two equal and opposite forces that produces a pure torque without any net force, resulting in pure rotation.
7. How do you determine if the moment of a couple is positive or negative?
The sign of the moment of a couple is determined by the direction of rotation it causes, following a standard convention in physics:
- A positive moment is one that causes a counter-clockwise (or anticlockwise) rotation.
- A negative moment is one that causes a clockwise rotation.
8. If both torque and work have the same dimensional formula [ML²T⁻²], how are they physically different?
Although torque and work share the same dimensional formula, they represent entirely different physical concepts. The main difference is that torque is a vector quantity that describes a turning force, calculated using a cross product (τ = r × F). In contrast, work is a scalar quantity that represents the transfer of energy, calculated using a dot product (W = F · d). Their identical dimensions are a coincidence of their defining formulas.
