An object is moving forward with a constant velocity. which statement about this object must be true?
a. no forces are acting on the object.
b. the net force on the object is in the forward direction.
c. the acceleration of the object is in the forward direction.
d. the net force on the object is zero.

If the object's velocity is constant ... (it's speed isn't changing AND it's moving in a straight line) ... then the net force on the object is zero. (D)

Either there are no forces at all acting on the object, OR there are forces on it but they're 'balanced' ... when you add up all of their sizes and directions, they just exactly cancel each other out, and they have the SAME EFFECT on the object as if there were no forces at all.

prndulum needs thread, weight clock to time swings