How to Calculate the Net Torque

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Torque is, simply put, the measurement of the force or forces that make an object rotate. Calculating net torque is a common exercise in physics classes, and it is usually taught during an introduction to rotational equilibrium. The SI unit for torque is newton-meter, abbreviated Nm. Torque is a vector quantity, so when calculating net torque, you have to take the directionality of all of the forces acting on the object into account, as well as the force itself.

    Decide upon an axis of rotation, sometimes called a pivot point. This is the point around which the object will rotate. In many simpler net torque problems, this will be the center of the object.

    Draw a diagram of the object and its axis of rotation. You will use this diagram throughout the process of calculating the net torque.

    Draw lines on the diagram to represent the forces acting on the object. Use arrow heads to keep track of which direction the force is being applied.

    Draw lines from the origins of the forces to the axis of rotation. These lines will be used as positioning vectors.

    Figure out the angle from the position vector to the force itself. In order to find the correct angle, use the right-hand rule: Pretend your hand is the positioning vector, and then curl your fingers toward the direction of the force -- the angle between your fingers and your hand is the angle you want.

    Calculate the moment arms of the forces. This is a line that runs perpendicular from the line of application, which you may need to extend, to the axis of rotation. This, along with the force line and the positioning vector, will make a triangle.

    Figure out the length of the moment arms by taking the length of the positioning vector and multiplying it by the sine of the angle you figured out in step five.

    Find each individual torque by multiplying the length of the moment arm times the force applied. Make sure to give each torque the proper sign: positive for forces resulting in counterclockwise movements, negative for forces resulting in clockwise movements.

    Add the torque values together. This will give you the net torque. If the answer is negative, the overall movement of the object will be clockwise. A positive number means counterclockwise movement.

    Tips

    • If the system is in equilibrium, set the net torque to 0 and work backward to find any other values.

References

About the Author

Michael Black has been a freelance writer based in South Central Pennsylvania since 2010. He graduated from York College of Pennsylvania with a Bachelor of Arts degree in professional writing. He has written music- and writing-related articles for various websites.

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  • pencil draw graph image by Anatoly Tiplyashin from Fotolia.com

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