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# AS mechanics help watch

1. I'm really struggling with knowing what to plug into suvat and to know whether to set things as positive or negative. like on this question:
A stone is thrown vertically downwards from a high building with initial velocity as 4ms-1. calculate the time required for the stone to travel 30m and its speed at this time.

So if i was filling in suvat, and i set downwards as positive, firstly would s = -30 or +30 or 0, and how would you know this?

For the next question:

A ball is thrown vertically upwards from the top of a cliff which is 50m high. the unitial velocity of the ball is 25ms-1. calculate the time taken to reach the bottom of the cliff and the speed of the ball at that instant.

So once again trying to fill in suvat and failing. if downwards is positive, do we use a = 9,81 or a = -9.81, and at what point are we looking at the acceleteration - at the end or start? also, do we set u as 25 or -25 as downwards is positive yet if ball is thrown upward it would be negative surely? also, would you set s as -50 or 50 and how ould you work this out.

Many thanks
2. (Original post by Bertybassett)
I'm really struggling with knowing what to plug into suvat and to know whether to set things as positive or negative. like on this question:
A stone is thrown vertically downwards from a high building with initial velocity as 4ms-1. calculate the time required for the stone to travel 30m and its speed at this time.

So if i was filling in suvat, and i set downwards as positive, firstly would s = -30 or +30 or 0, and how would you know this?

For the next question:

A ball is thrown vertically upwards from the top of a cliff which is 50m high. the unitial velocity of the ball is 25ms-1. calculate the time taken to reach the bottom of the cliff and the speed of the ball at that instant.

So once again trying to fill in suvat and failing. if downwards is positive, do we use a = 9,81 or a = -9.81, and at what point are we looking at the acceleteration - at the end or start? also, do we set u as 25 or -25 as downwards is positive yet if ball is thrown upward it would be negative surely? also, would you set s as -50 or 50 and how ould you work this out.

Many thanks
For the first one, displacement is positive. Displacement is almost always positive, and I've personally never seen a question that requires negative displacement. Although they probably do exist for some questions where you have displacement as a function of time, but you don't have to worry about that here.

For the second one, first find the upwards displacement, which uses the negative acceleration of free fall (-9.81), because it has a direction OPPOSITE to the direction acceleration of g (towards the center of the Earth). Then add that height to the 50m, so you have the total vertical displacement from the ground (STILL positive). Then simply use g as 9.81, u as 0, and calculate the time (using S=ut+1/2at^2 as you don't know the final velocity).

Also, unless otherwise stated, acceleration is usually uniform (constant). The rate of change of acceleration (second derivative of velocity) would have to be given, if it wasn't uniform, which you won't need to worry about. Acceleration of free fall (the value of g) never changes (except negligible effects of terrain, but pretty irrelevant).
3. (Original post by RedGiant)
For the first one, displacement is positive. Displacement is almost always positive, and I've personally never seen a question that requires negative displacement. Although they probably do exist for some questions where you have displacement as a function of time, but you don't have to worry about that here.

For the second one, first find the upwards displacement, which uses the negative acceleration of free fall (-9.81), because it is OPPOSITE to the acceleration of g (towards the center of the Earth). Then add that height to the 50m, so you have the total vertical displacement from the ground (STILL positive). Then simply use g as 9.81, u as 0, and calculate the time (using S=ut+1/2at^2 as you don't know the final velocity)
thanks for the reply but for the first one would s = 0 or 30 then, as how do you know if it's in relation to the ground? and im just lost on what u mean on the second one
4. (Original post by Bertybassett)
thanks for the reply but for the first one would s = 0 or 30 then, as how do you know if it's in relation to the ground? and im just lost on what u mean on the second one
Displacement is 30. Just think of it as the magnitude of the displacement (the distance).

You have two stages for the second question (I don't know if you know or not). Find the vertical height that the object is thrown (acceleration, g, is negative). Then get the TOTAL displacement from the ground (the height the object is from the ground) by adding the height you just calculated to the 50m. For this, acceleration, g, is positive. Draw out a diagram if you're not sure.

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