I think I understand what the problem was: I wasn't sure about which "system" to consider in my calculations. Take the table and object example for instance. There's a force opposing the motion of the block on the table, because of the contact of the block with the table. Now in this case we're...
If an object is moving forwards on a rough plane, we assume that there is a force opposing its motion, namely friction or resistance. This friction includes the friction because of the contact between the moving object and the ground. From Newton's third law, we can conclude that because the...