Abzdot
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Hi, I just need a quick hand with the signs you use with suvat for objects under free fall like a particle being launched into the air.
I take down as positive and up as negative. This means that if the particle is launched up off a platform of X metres this would be negative with an initial velocity of U I would take it as negative with g as positive. However, displacement is actually not negative but is positive. Why is this?
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ghostwalker
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(Original post by Abzdot)
Hi, I just need a quick hand with the signs you use with suvat for objects under free fall like a particle being launched into the air.
I take down as positive and up as negative. This means that if the particle is launched up off a platform of X metres this would be negative with an initial velocity of U I would take it as negative with g as positive. However, displacement is actually not negative but is positive. Why is this?
The point of projection is s=0. If the platform is at a height of X metres, then when the particle lands it will be X metres below the platform. If you're taking down as positive, then this is a positive displacement.
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Pangol
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(Original post by Abzdot)
Hi, I just need a quick hand with the signs you use with suvat for objects under free fall like a particle being launched into the air.
I take down as positive and up as negative. This means that if the particle is launched up off a platform of X metres this would be negative with an initial velocity of U I would take it as negative with g as positive. However, displacement is actually not negative but is positive. Why is this?
Under that convention, a positive displacement means that the object has ended up lower down than it started. Does this fit with the question you are doing?

There is no rule that says which way you have to make positive, you can decide from question to question. It's usually easier to make positive the direction of the initial motion, so I would do it the other way around in this case.
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