Chronology | Current Month | Current Thread | Current Date |
[Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
This is a one dimensional problem
An object accelerates if it speeds up, slows down, or changes direction.
In one dimension, an object that speeds up has an acceleration in the
same direction as the motion while an object that slows down has an
acceleration opposite the direction of motion. Now for the vertically
thrown ball, on the way up it is slowing down, therefore the
acceleration is opposite the direction of motion--downwards. On the way
down the ball is speeding up, therefore the acceleration is in the
direction of motion--downwards. The acceleration is always downwards.
[Now convince them it is still accelerating at the peak of the motion.]
You've defined DOWN as the negative direction in this problem.
Therefore since the acceleration is always down, it must always have a
negative value.
Of course you could (and should at times) define down
to be the positive direction. The ball still accelerates downwards, but
now 'g' would carry a positive sign.