7. Jamal is rolling along on his 4.0 kg skateboard with a constant speed of 3.0 m/s when he jumps off the back and continues forward with a velocity of 2.0 m/s relative to the ground. This
causes the skateboard to go flying forward with a speed of 15.5 m/s relative to the ground.
What is Jamal's 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.

An electromagnet is a material in which the current produces electromagnetism and is passed to the core.  so to fill in the blanks, choices would be better if there were.an electromagnet is a magnet produced by electric current with a metal core