The current in the half-loop goes in the negative x direction and also goes in the positive y direction and then the negative y direction.
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The B field in the z direction applies force to current in x direction and current in y direction. But here's the trick:
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Since the current goes in the positive and then negative y directions, the force on the current in the y direction cancels out. The total force on the "y current" is zero!
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This means that the total force on the half-circle is only on the current that moves in the x direction. The total length of the "x current is just 2R (i.e. the distance the current moves in the x direction). So now...
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force on current in B field = L I B = 2 R I B = 2 * 4 * 2 * 3 = 48 N
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And the direction? This is found using the right hand rule. The direction of force is the direction of current (cross) B field, which in your case is negative x (cross) z . The result of this is positive y.
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So total force on the half-circle is 48 N in the +y direction