Torque is the scientific term for a twisting or rotational force, representing how effectively a force can cause an object to turn around an axis or pivot point. It is the rotational equivalent of the linear force that causes an object to move in a straight line. When this twisting force is applied to an object, such as turning a wrench or spinning a motor shaft, an equal and opposite twisting force pushes back. This opposing force is known as reaction torque, and it is an unavoidable part of any system involving rotation.
Defining Applied Torque vs. Reaction Torque
The concept of torque in any system must be considered as a pair, distinguishing between the intentional action and the resulting consequence. Applied torque is the rotational force an individual or a machine purposefully exerts on an object to cause a change in its angular momentum or state of rotation. This force might be generated by a hand turning a doorknob, a motor spinning a drill bit, or an engine rotating a driveshaft.
Reaction torque is the rotational force that the object or system exerts back against the source of the applied torque. This reaction is always equal in magnitude and opposite in direction to the applied torque. For example, if a motor applies 50 Newton-meters of clockwise torque to spin a shaft, the mounts holding that motor must absorb an opposing 50 Newton-meters of counter-clockwise reaction torque.
The relationship between applied and reaction torque is a continuous push-pull dynamic existing whenever a rotational effort is made. When a person pushes a stationary wall, the applied force is the action, and the wall pushing back with the same force is the reaction. Similarly, applied torque is the action, and reaction torque is the immediate, simultaneous response. This opposing force ensures that the overall rotational energy of the system is conserved, even if the object being twisted is not moving.
The Physics Behind Reaction Torque
The existence of reaction torque is a direct consequence of the rotational analogue of Newton’s Third Law of Motion. When one body, such as a motor, applies a torque (action) to a second body, the second body simultaneously applies an equal and oppositely directed torque (reaction) back onto the first body. This principle applies just as much to twisting forces as it does to linear pushes and pulls.
In a rotating system, reaction torque is necessary to maintain rotational equilibrium. Rotational equilibrium occurs when the net torque acting on a system is zero, meaning the object is either stationary or rotating at a constant speed. When an external force introduces an applied torque, the reaction torque instantly arises to oppose it. If the system is not rigidly anchored, the body generating the torque will experience an angular acceleration in the opposite direction.
The magnitude of the reaction torque is determined by the applied torque, which is calculated as the force multiplied by the perpendicular distance from the axis of rotation. This principle explains why securing a rotating machine is important; the machine’s connection to the ground or frame is the anchor that absorbs the reaction torque. Without a rigid anchor, the reaction torque acts on the machine’s body, attempting to rotate it in the opposite direction of the intended output.
Everyday Scenarios of Reaction Torque
Reaction torque is a common experience, even in simple household tasks. Consider the effort required to open a stubborn jar lid that has been sealed too tightly. As you apply an opening torque with your hand to the lid, the lid simultaneously applies an equal amount of counter-clockwise reaction torque to your wrist. If the lid does not budge, your hand and arm are absorbing all of the reaction force.
Another common example occurs when using a manual screwdriver to fasten a screw into a dense material like hardwood. The applied torque twists the screw into the wood, but the resulting reaction torque attempts to spin the handle back out of your grasp. This opposing force requires you to grip the handle tightly to prevent the screwdriver from rotating in your hand.
Even the act of stirring a thick batter or dough with a spoon demonstrates this principle. As you apply a rotational torque to the spoon to move the viscous material, the resistance of the batter generates an equal and opposite reaction torque on your hand and arm. The thicker the mixture, the greater the resistance, which translates directly into a larger reaction torque you must resist to continue stirring.
Practical Implications in Tools and Machinery
In engineering, managing reaction torque is a design consideration for safety, efficiency, and system integrity. In power tools, such as an electric drill or impact wrench, the motor applies a high level of torque to the fastener or drill bit. The resulting reaction torque causes the tool’s body to attempt to rotate in the opposite direction, a phenomenon commonly known as “kickback” that the user must brace against. Impact drivers mitigate this force through an internal hammering mechanism that delivers torque in rapid, short bursts, reducing the continuous reaction force applied to the operator’s wrist.
In vehicle mechanics, the engine produces torque to turn the wheels, and the reaction torque attempts to rotate the entire engine block around its centerline. This rotational force is absorbed by the engine mounts, which are designed to withstand this twisting action and prevent the engine from rotating within the chassis. If the mounts were to fail, the engine casing would be free to rotate in response to the torque it is producing.
Reaction torque management is found in helicopters, where the main rotor applies torque to the air to create lift. The resulting reaction torque on the fuselage is powerful enough to spin the entire aircraft in the opposite direction. To counteract this, a smaller, vertically mounted tail rotor is used to generate a thrust that creates an opposing torque, ensuring the helicopter’s body remains stable and oriented correctly.