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/sci/ - Science & Math

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4657336 No.4657336 [Reply] [Original]

In a time equation of a position like :
X(t) = X0 + S0*t ± 1/2 * g * (t)²

How do I know if I have to add or substract 1/2 *g*(t)²?

It depends on which parameters?

>> No.4657352

thanks

>> No.4657365

i think its accounting for the fact that g can be positive or negative, the formula should use a +

>> No.4657370
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4657370

>>4657365
and then again how do I know if G is + or -?

>> No.4657384

Depends on how you set up your coordinate system? If your position equation describes e.g. a ball thrown upwards and s is measured upwards increasing, g works against it's upwards motion, so it is negative.

>> No.4657397
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4657397

>>4657384
allright, I get it now thanks a lot

>> No.4657746

General rule is to pick co-ordinates which increase in direction of force/accn vector. Don't have to. Just tends to be bit easier.

This is why g is +ve by convention. From this convention, ground level is, say x=0. And a rooftop would be e.g -100m.