Mass - hard to change the motion
Teaching Guidance for 14-16
Some parts of you are harder to shift than others – and that's not all in the mind
Remember that, in getting parts of your body moving, it's not only the mass – there's bound to be something about levers in there somewhere as well. The complexity of the situation suggests that using body parts may not be a good idea when introducing inertial effects.
Don't use parts of the body, or really anything that doesn't act as a point mass (so where you need to think about the internal structure), to introduce inertial mass and its effects. Concentrate on simple objects.