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# SUVAT....confused.... watch

1. Hi, I'm currently revising M1 and even though I know this is the most basic chapter, I'm kinda confused about SUVAT.

Every time I do a question I get the signs wrong. I know that u, v, a and s are all affected by direction.

E.g. At time t = 0 a ball is projected vertically upwards from a point O and rises to a maximum
height of 40 m above O. The ball is modelled as a particle moving freely under gravity.
Show that the speed of projection is 28 m s–1.

So if I take up as the positive direction, then I would make s, a, v, positive if the particle is moving upwards, expecting u to be positive. I did this and u is negative which I just don't understand.

Thanks for the help!
2. (Original post by djels013.211)
Hi, I'm currently revising M1 and even though I know this is the most basic chapter, I'm kinda confused about SUVAT.

Every time I do a question I get the signs wrong. I know that u, v, a and s are all affected by direction.

E.g. At time t = 0 a ball is projected vertically upwards from a point O and rises to a maximum
height of 40 m above O. The ball is modelled as a particle moving freely under gravity.
Show that the speed of projection is 28 m s–1.

So if I take up as the positive direction, then I would make s, a, v, positive if the particle is moving upwards, expecting u to be positive. I did this and u is negative which I just don't understand.

Thanks for the help!
Even though the particle is moving upwards, its acceleration is pointing downwards (due to gravity) which is why it will slow down as it rises.

You need to get used to the difference between velocity and acceleration . Velocity can be one way while acceleration can be the opposite way.

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