Question

What is the moment of force and moment of a couple?

Answer 1

Hint:We must know the difference between moment of force and moment of couple. Moment of force is responsible for making an object rotate about an axis. Depending on the position of the axis and the distance of the force acting, the value of moment varies. A couple is a system of forces with a resultant moment but no resultant force.

Complete step-by-step solution:
The moment of force is also better known as the torque or simply the moment. It determines the tendency of a force to rotate the body of application about a specific point or axis of rotation. Since force is a vector quantity and so is the radial distance between the point of application of the force and the pivot, the moment of force is calculated by taking the cross product of the two. Its direction is determined by using the right hand thumb rule.
 The couple is a pseudo-vector quantity, which has both magnitude and direction. The magnitude of the net couple is given by taking the net torque from taking the clockwise or anticlockwise moments which are acting on any arbitrary point of the rotation. The direction of the couple is given by the right hand screw rule. If the thumb points the direction of force, then the curling of the fingers give the direction of the torque. Also, since the couple is a cross product of the force and the perpendicular distance, the couple is perpendicular to the plane consisting both the force and the perpendicular distance. The SI unit of the couple is N-m.

Note:An example of moment of force or torque is hinged door. To open the door, we push or pull the farthest end from the hinge point. This causes the rotational motion of the door and it opens. If we push very close to the hinge point, it would require us to use a lot of force and still the force may not open. A couple is a special case of moment, where two anti-parallel torques separated by a distance d.