'Intelligent person' could be a possibility for your answer, also a 'scientist' or a 'philosopher' as well as 'an old person' may equal to the meaning of being 'wise humans'.
Air is the answer i do believe
If you double the velocity you would have to halve the magnetic field strength to keep the force on the particle the same and hence the trajectory.
F = qvB
Answer:

Explanation:
Needed torque can be estimated by means of the Theorem of Angular Momentum Conservation and Impact Theorem. The center of mass of the system is:


Let assume that both masses can be modelled as particles, then:
![[(1.51\,kg)\cdot (0.923\,m)^{2} + (1.97\,kg)\cdot (0.707\,m)^{2}]\cdot (38\,\frac{rev}{min} )\cdot (\frac{2\pi\,rad}{1\,rev} )\cdot (\frac{1\,min}{60\,s} ) -T\cdot (7.5\,s) = 0\,\frac{kg\cdot m^{2}}{s}](https://tex.z-dn.net/?f=%5B%281.51%5C%2Ckg%29%5Ccdot%20%280.923%5C%2Cm%29%5E%7B2%7D%20%2B%20%281.97%5C%2Ckg%29%5Ccdot%20%280.707%5C%2Cm%29%5E%7B2%7D%5D%5Ccdot%20%2838%5C%2C%5Cfrac%7Brev%7D%7Bmin%7D%20%29%5Ccdot%20%28%5Cfrac%7B2%5Cpi%5C%2Crad%7D%7B1%5C%2Crev%7D%20%29%5Ccdot%20%28%5Cfrac%7B1%5C%2Cmin%7D%7B60%5C%2Cs%7D%20%29%20-T%5Ccdot%20%287.5%5C%2Cs%29%20%3D%200%5C%2C%5Cfrac%7Bkg%5Ccdot%20m%5E%7B2%7D%7D%7Bs%7D)
The torque needed to stop the system is:
