Acar is traveling at 60 mph on cruise control. what is the net force?

The net force is zero.

According to Newton's first law, every body continues in its state of rest or in a state  of uniform motion in a constant direction, unless acted upon by an external unbalanced force. This law leads to the principle that no force is required to keep the body moving with a constant velocity.

According to Newton's second law, force acting on a body is proportional to the rate of change of momentum produced in the body. In simple terms, the force F acting on a body is given by the product of its mass and the acceleration produced in it. Thus, when a force acts on a body, the body is accelerated.

A body is said to be accelerated, if its speed changes or its direction changes or if both its speed and its direction changes.

Since the car in the question is travelling at a constant speed of 60 mph, it is not accelerated, hence the total external force acting on the car is zero.

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it