A bar of length 2L can rotate about a frictionless axle at its center. The bar is initially at rest and is then acted on by the three forces shown. What is the magnitude of the net torque on the bar?

Did the give any angles?

To determine the magnitude of the net torque on the bar, we need to consider the individual torques produced by each force.

The torque produced by a force can be calculated using the equation:
τ = r * F * sin(θ)

Where:
τ is the torque (moment)
r is the perpendicular distance from the axis of rotation to the line of action of the force
F is the magnitude of the force
θ is the angle between the force vector and a line perpendicular to the bar (in this case, the lever arm)

Since the bar can rotate about a frictionless axle at its center, the perpendicular distance from the axis of rotation to the line of action of each force is L.

Now let's calculate the torques produced by each force:

1. The torque produced by the force F1 can be calculated as:
τ1 = L * F1 * sin(θ1)

2. The torque produced by the force F2 can be calculated as:
τ2 = L * F2 * sin(θ2)

3. The torque produced by the force F3 can be calculated as:
τ3 = L * F3 * sin(θ3)

Finally, the magnitude of the net torque is the sum of the individual torques:
net torque = |τ1| + |τ2| + |τ3|

Please provide the magnitudes of the forces (F1, F2, F3) and the angles (θ1, θ2, θ3) between the force vectors and the perpendicular line to the bar, and I can help you calculate the net torque.