Final answer:
Torque is calculated using the formula T = rF sin θ. When the force is perpendicular to the radius, this simplifies to T = rF. In the case of a rectangular loop in a magnetic field, only the side segments contribute to the torque.
Step-by-step explanation:
To calculate the torque (T) applied to a rectangular tube or any other object, we use the formula T = rF sin θ, where T is the torque, r is the distance from the pivot point to the point where the force is applied, F is the magnitude of the force, and θ is the angle between the force and the lever arm. If the force is applied perpendicular to the lever arm, as indicated, sin θ is equal to 1, and the equation simplifies to T = rF.
In situations where we have a rectangular loop in a uniform magnetic field, and we are looking to find the torque on the loop, the top and bottom segments of the loop exert no torque since the forces are parallel to the axis of rotation. The sides of the loop, however, do contribute to torque, and if the force is perpendicular to these side segments and r is half of the width w of the loop, then each side contributes a torque of (w/2)F, adding up to the total torque.