Given that, no field is acting on the electron. Therefore, net force on the particle is equal to zero.
As the cork is floating on water, it's weight is being balanced by the upthrust which is equal to the weight of the water displaced. Hence, net force acting on the cork is zero.
Given, the velocity with which the car is moving is constant. So, acceleration=0. Hence, net force on the car is zero.
As the raindrop is falling with a constant speed, acceleration = 0. Hence, net force ma= 0.