Describe a situation in which an object is not in equilibrium, even though the net force on it is zero.

Newton's second law says any net force on an object results in acceleration, F=ma. So anything not accelerating has no net force. Consider compressing a spring with your hands and holding it still. You push on the spring to hold it out of its natural position (out of equilibrium) but it is not accelerating so the net force is zero. It's pushing back on you with the same force as you are pushing on it.