Again, this is a picture of a tennis ball.  Last time I used this picture I talked about it rolling and it's velocity. This time, I am going to talk about how it is displaying Newton's first law, the law of inertia.  Newton's first law states that objects in motion tend to stay in motion unless acted upon by an outside unbalanced force.  Likewise, objects at rest tend to stay at rest unless acted upon by an outside unbalanced force.  This ball is currently in equilibrium.  I know this because it is not moving or more precisely, it is not accelerating.  Because it is not in motion, the ball will not move until an outside force, like me for example, moves it.  The more mass the object has, the more it has a tendency to stay inert.  This is Newton's first law.