Consider the specific example of a positive charge q moving in the +x direction with the local magnetic field in the +y direction. In which direction is the magnetic force acting on the particle?

Explanation:

The positive charge is moving in the positive direction, this shows that the velocity of the charges is in the positive direction where the charge is moving,

Therefore,

V is in the positive x-direction

V=Vi

The magnetic field is in the positive y-direction

B=Bj

Then,

Force in magnetic field is given as

F=q V × B

Where V and B are cross products

But note that

i×i=j×j=k×k=0

Also,

i×j=k, j×k=i, k×i=j, j×i=-k, k×j=-i, i×k=-j

Then, now applying this,

F= q V × B

F= q V i × B j

Then, since i×j=k

F=qVB •k

Then, the magnetic force is pointing in the positive z direction.

Check attachment for the directions

2. they have negligible mass.

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.