sometimes referred to as the law of inertia. An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton's 2nd law
*Both are going 5 mph*
Newton's 3rd Law
The tree was at a constant rate of zero, until the unbalaned force from the wind made it move.
Both people are accelerating to the same amount but because on person has more mass on it, it goes slower.