I worked on collision problems, many had the use of non constant forces, like a chain falling onto a scale, or marbles dropping into a box, hail stones at a certain frequency falling, or whatever other collisions. I couldn’t solve the chain one myself, so I spent a while studying the solution. But I got another similar problem right, so hope that means I understand it. Variable mass systems are actually very interesting to me, especially how they deviate from the typical high school physics problems.