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SUVAT -help with understanding.

This might be a really stupid question but with the equation s=ut + 1/2at^2 when would u use a + and when would u use a - ( so it would be s=ut-1/2at^2)??
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Pangol
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Original post by r.rosex
This might be a really stupid question but with the equation s=ut + 1/2at^2 when would u use a + and when would u use a - ( so it would be s=ut-1/2at^2)??


The - version is slightly different: s = vt - 1/2at2 (so that's a v in this one, not a u).

This makes it easier to see which one you would use depending on your situation. If you know three of s, u, a and t and want to know the other, use the first version. If you know three of s, v, a and t and want to know the other, use the second.
Original post by r.rosex
This might be a really stupid question but with the equation s=ut + 1/2at^2 when would u use a + and when would u use a - ( so it would be s=ut-1/2at^2)??

An alternative take on this is that it's always s=ut + 1/2at^2, but that the value of a may sometimes be negative. The way to decide whether a is positive or negative is:
1) Decide which direction is positive for the question in hand, and use that direction consistently. For example UP is positive.
2) If the object is accelerating in the positive direction (equivalent to decelerating in the negative direction), then a is positive for suvat purposes. If the object is decelerating in the positive direction (equivalent to accelerating in the negative direction) then a is negative for suvat purposes.

Consider an object thrown vertically upwards with speed u, and moving freely under the influence of gravity. If we choose upwards as the positive direction, then we must take u as positive and a as negative (-9.8). Likewise a positive value of displacement s means an upwards displacement. Alternatively if we take downwards as the positive direction then then we must take u as negative and a as positive (+9.8). Likewise a positive value of displacement s means a downwards displacement.

Hope this helps.

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