Here's an odd experiment. I strung two sets of masses together with a piece of string. It's 400 grams on the left and 600 grams on the right. The 400 grams is in mid air because of the greater gravitational attraction of the 600 grams. Then I set the 400 grams swinging. Suddenly the 600 grams started to lift and fall, lift and fall. This shows that the force exerted by the 400 grams when it swinging is bigger than the 4 newtons exerted when it is still. When an object moves in a circle, there needs to be a centripetal force acting towards the centre of turning. A centripetal force is not a force in its own right: it is a resultant force, a combination of other forces. In this case, the centripetal force = tension in string - 4N. The 4 newtons cannot change, so to get a centripetal force the tension has to increase. It increases so much that it can lift the 600 grams off the ground momentarily. So if you are designing a swing, the cables need to be enough to support it when it is moving.
