De-celleration because speed is lower as time goes on

The best method to use in prevention of self-demagnetization of a permanent magnet is by use of a keeper. Keepers were also commonly known as amateurs. They help in storing magnets safely, especially those that have low coercivity ( magnets that are extremely susceptible to stray fields).

Hope It Helps!
This is mr day and infact am seeing this… come into my office after class.
I am not sure of the answer but I think that the answer is a
Use Fnet*t=p
Fnet=mv2-mv1÷t
Fnet=3(12)-3(0)÷1.5
Fnet=36÷1.5
Punch this intoba calculator and get your answer. S.I unit = N