Newtons First Law

The pennies were at rest upon Derek's elbow. As Derek pulls his elbow from underneath the pennies the acceleraion is is equal to gravity.  The only force that is acting on the pennies is gravity because Derek's elbow is just being pulled from underneath the three stacks.  The hard part to catching these pennies is that they as you try to catch them, they are accelaerating towards the ground for a fraction of a second.  The only way these object will stop is if they Dereks hand (a force) acts upon them, or they will hit the ground. Inertia means that an object in motion like the pennies, or an obect not in motion, will stay in that motion until they are acted upon by some force such as the ground.