Notice that in this case, F and g are not conventionally written as vectors, because they are always pointing in the same direction, down. In this case, the constant acceleration due to gravity is written as g, and Newton's Second Law becomes F = mg. There is one situation, however, in which we do encounter a constant force - the force due to gravitational acceleration, which causes massive bodies to exert a downward force on the Earth. The body might speed up, slow down or change direction, after which, the body will continue moving at a new constant velocity (unless, of course, the impulse causes the body to stop). For a massive body moving in an inertial reference frame without any other forces such as friction acting on it, a certain impulse will cause a certain change in its velocity. In most cases, forces can only be applied for a limited time, producing what is called impulse. It is rather difficult to imagine applying a constant force to a body for an indefinite length of time. The large Σ (the Greek letter sigma) represents the vector sum of all the forces, or the net force, acting on a body. In this case, we would write the equation as ∑ F = m a The force can be a single force or it can be the combination of more than one force. The bold letters F and a in the equation indicate that force and acceleration are vector quantities, which means they have both magnitude and direction. However, if the object is already in motion, or if this situation is viewed from a moving inertial reference frame, that body might appear to speed up, slow down, or change direction depending on the direction of the force and the directions that the object and reference frame are moving relative to each other. In the simplest case, a force applied to an object at rest causes it to accelerate in the direction of the force. Newton's second law says that when a constant force acts on a massive body, it causes it to accelerate, i.e., to change its velocity, at a constant rate.
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