Question

This question is relating free fall(both upward and downward motion).

for downward motion we take g=+9.8m/s^2, and for upward motion, we take it as a=-9.8m/s^2.
The question is as the acceleration(both direction and magnitude) is towards the centre of the earth in both of the cases i.e downwards; then why do we take +ve in one case and -ve in another case?

If the acceleration is towards the earth only, then why are the symbols different in two nearly similar situations?

Answer 1

Answer::

Depends which way you orient your axes.

This is a totally arbitrary coordinate selection — it makes no difference to the physics involved.

If you decide that you want “up” to be a positive value, then g≈−9.8 ms^−2 . This is the generally accepted standard.

However, it is equally valid to state that you want to orient your coordinates such that “falling” is positive, in which case g≈ 9.8 ms^−2 .

It makes no real difference —you just need to make sure that you are consistent with this choice throughout your working. If you use “up” as positive and g≈+9.8 ms^−2 , then you will get silly answers, such as balls flying off into space when you drop them.

Answer 2

Lateral Inversions in Plane Mirrors

The word ‘AMBULANCE’ is opposite because when the driver of the vehicle ahead of the ambulance looks in her/his mirror, the person can read it as ‘AMBULANCE’ and give way to it.

Thanks