What equation gives the position at a specific time for an object with constant acceleration?  

Given constant acceleration, we can get the final position of an object in terms of both its initial velocity and its acceleration using one of the equations of motion.

The equation that we will use is:
Xf = Xi +  Vi*t + (1/2)*a*t^2
where:
Xf is the final position of the object
Xi is the initial position of the object
Vi is the initial velocity of the object
t is the time
a is the constant given acceleration

2. they have negligible mass.

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