It's interesting because I would have logically considered those things to be exactly backwards.
In actual physics terms, force is timeless. f = m a
In physics engines, I'd expect this to be multiplied by time like any other acceleration.... but have mass removed. I often find that mass must be premultiplied "out" of the force. For example, in my physics engines I think I treat force just like another acceleration/velocity accumulator.
An impulse is a force over a specific time. So impulse = f * t
A force would be being dragged by a car. Every frame that the force is active, you are being pulled forward.
An impulse would be being kicked in the back. It's a one time force over a specific part of a second.
I'm curious where impulse is mentioned in dyn4j as it never came up for me. Because physics engines are already dealing with time slices, force and impulse are often interchangeable. A force applied this frame and not the next was a tpf-length impulse.