All objects in free fall accelerate at?

Question

a) 9.81 m/s

b) -9.81 m/s^2

c) 0 m/s^2

d) 9.81 m/s^2

e) -9.81 m/s

Answer 1

If you take the upward direction as positive, then the free-fall acceleration is

 

a = -g = -9.81 m/s²

 

Both b) and d) have the correct magnitude of the acceleration, but b) accounts for the acceleration being downward if the upward direction is defined as positive.  However, you could just as easily have defined the downward direction as positive, in which case d) is the correct answer.  The sign of the answer depends on which direction is defined as positive.  The problem statement does not define the positive direction, so there is some ambiguity.  That is one of the problems with multiple choice questions.  I suggest always defining up as positive, hence free-fall acceleration is negative, and b) is the best answer.