Why did you need a permanent magnet? What do you think would happen if you increased the number of permanent magnets? (dc motor)

Permanent magnets are important for their industrial uses especially when it comes to power generation and electric motors. The induction process for turbines and generators needs permanent magnets to turn mechanical motion into energy.

Exposing magnetized magnets to opposing external fields produces a demagnetizing effect. This effect is most apparent in permanent magnet materials having a non-linear demagnetization curve, such as Alnico. ... Other factors such as SHOCK and VIBRATION have very little effect on today's permanent magnet materials.

answer: 2×10^-2kg

explanation: density= mass per volume

vol=mass/density

converting to s.i units we have;

3÷1000kg = 0.003

1.5÷100m = 0.15

0.003/0.15 = 0.02kg

0.93xc=0.93x3x10^8m/s

2.54cm is 1 inch

360seconds i 1 hr