Direction of gravity in Projectile (1 Viewer)

[sugaryblue]

I have been having problems with this since the beginning. in those projectile equations, when do I know to use -ve sign or +ve sign for gravity?

 

[kini mini]

Define the postitive direction yourself at the beginning of the question. Sometimes it's easier to do it one way than the other, so be prepared to change if necessary .

 

[sugaryblue]

Originally posted by kini mini
Define the postitive direction yourself at the beginning of the question. Sometimes it's easier to do it one way than the other, so be prepared to change if necessary .

so, let's say, if I've put going down as being negative, does that mean that I have to put gravity as negative?

If so, is there anything other signs I need to change??


Thanks

 

[kini mini]

Originally posted by sugaryblue
so, let's say, if I've put going down as being negative, does that mean that I have to put gravity as negative?

If so, is there anything other signs I need to change??

If gravity is acting downwards in the context of your problem, yes .

You might have to change things if you have memorised formulae that make asumptions about vectors...but as you have to prove everything in this topic IIRC just go through carefully and logically .

At worst, just do it the same way every time .

 

[inasero]

as kini min says, it doesnt matter which way you have positive and negative as long as you stick to one system and remain consistent...also note that gravity itself does not have a direction, only the FORCE (influence) doe to gravity has a direction....and this is always downwards towards the centre of the earth

 

[gnrlies]

If gravity is speeding the object up, then keep it as positive. If gravity is slowing it down, then take it as negative (its the same as accelleration as being positive and decelleration being negative). It just depends on the situation. So if something is going upwards, then gravity would be a decelerating force as it is acting against the object, howevor if you were to drop something then gravity would be accelerating the object.