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Grade: 12

Dear experts,

While going through Resnik & Halliday's book on Physics, I found the free fall acceleration section a bit confusing. We had been taught in earlier classes(at school) that the value of "g" for equations of uniformly accelerated motion would depend on the object going upward or downward. An object going upward would have a negative g, and one downward a positive g.

But in the book, they have explained that the value of g is always taken negative(g = -9.8 m/s2). They say that the correct way to interpret the sign of acceleration is that if the signs of acceleration and velocity match, the speed of a particle increases and if they differ, speed decreases. Which convention am I supposed to follow??

11 years ago

Answers : (3)

Prashant Sahay
19 Points

See basically what is important is to understand that

so direction of force is the same as direction of accln and direction of accln is same as that of "change in velocity".But direction of accln and velocity need not be the same.If it is then the body is under accln and if it is opposite  then it is retardation.

And since gravitational force acts towards center of the earth , so it is better to take the direction of accln as downward and accordingly your sign scheme works.


11 years ago
Pritish Chakraborty
8 Points

Ahh I understand. They also did mention a coordinate Y axis. So since g is always downward, it will be taken negative. Thanks for the help!

11 years ago
XYZ Person
28 Points

Dear Pritish,

follow wat had been taught to  u coz it had been the convention u had been following from school. 

also wat matters is whether u got correct ans or not ,its not about d sign convention(even in geometrical optics u will hav 2 diff sign conv) 

9 years ago
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