This gives me flashbacks I got so confused by this question, and my teacher couldnt even figure it out at first
First, the simple part. For the object to stay in circular motion, moved solely by the magnetic field, the centripetal force must be equal to the magnetic force.
But, if you just substitute these values into the equation, you’ll get 0.0159, which isn’t an answer. The reason for this is that the objects velocity is close enough to the speed of light that we have to consider special relativity, so you have to consider the length contraction which occurs to the radius using the formula on ur ref sheet (you might not have learnt this yet), and you should get r=74.55m using that. Then, you can sub it into the formula for magnetic field and find the answer to be 0.0175T