That's at the highest point of the swing... whether it's in front or in the back ...
at the instant when you stop going up and start coming down.
For the answer to the question above, we are given with two isotopes of oxygen, one that is stable O-18 and O-19 that is unstable. The stability of a nucleus is relative to the nuclear binding energy and mass defect. In this case, nuclear binding energy is always. So the answer is <span>O-19 has a mass defect; O-18 does not have a mass defect.</span>