Hello, Please refer to the below link. This is IIT foundation book for 10th grade Physics. I wonder why the sign for g is negative both for upwards and downward direction of motion? Thanks in advance for your time and response. http://books.google.co.in/books?id=xTiC3gHKc1MC&pg=PA8&dq=according+to+sing+convention,+displacement,+velocity+and+acceleration,+being+vector+quantities&hl=en&sa=X&ei=NpJFUf-7Dcm4rAeC2oDgCA&ved=0CDEQ6AEwAA#v=onepage&q&f=false

according to sign conventions upward direction is taken as positive and downward direction is taken as negative . g is always downwards and therefore taken as negative.in downward motion displacement will be in downward direction and therfore displacement ,velocity ,and g all will be negative.while moving downwards magnitude of velocity will increase and beccome more negative.

its not compulsory to take g negative both up and down.but since many students make a mistake about the sign of g,so a sign convention is followed in which the vectors pointing upwards are taken positive while those pointing downwards are taken negative.as a result g is taken negative since it acts in downward.

hope i helped u.

Thanks for your reply. Will you please clarify further on your point that g need not have to be taken -ve both upwards and downwards. My understanding is that g always acts downwards, so will always have one sign either +ve or -ve depending on what sign we assign to other downward acting forces.