# A1 mechanic question

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#1
is this right?
so if an object goes up it will have a - 9.8 the V=0 and U= (a value)
but if a object falls down it will have 9.8 the U=0 and V = (a value )
0
4 weeks ago
#2
in most cases yes. you just need to make sure for the 2nd one that it will have an initial velocity of whatever is given in the question.
1
4 weeks ago
#3
(Original post by interlanken-fall)
is this right?
so if an object goes up it will have a - 9.8 the V=0 and U= (a value)
but if a object falls down it will have 9.8 the U=0 and V = (a value )
If you are asking this, then you are missing the fundamental idea you are free to choose which direction you choose to be the positive one.

If upwards is your positive direction, then g=-9.8 since it acts in the opposite direction.
If downwards is your positive direction, then g=9.8 since it acts in the same direction.

The choice is up to you to make. One setup is more motivated than the other depending on the problem.

E.g. if an object is falling down then you may want to say downwards is your positive direction in which case a=9.8, but there is no problem going the other way and sticking to a=-9.8.
Last edited by RDKGames; 4 weeks ago
2
4 weeks ago
#4
(Original post by RDKGames)
If you are asking this, then you are missing the fundamental idea you are free to choose which direction you choose to be the positive one.

If upwards is your positive direction, then g=-9.8 since it acts in the opposite direction.
If downwards is your positive direction, then g=9.8 since it acts in the same direction.

The choice is up to you to make. One setup is more motivated than the other depending on the problem.

E.g. if an object is falling down then you may want to say downwards is your positive direction in which case a=9.8, but there is no problem going the other way and sticking to a=-9.8.
damn moderators are smart
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#5
(Original post by Ashprit)
in most cases yes. you just need to make sure for the 2nd one that it will have an initial velocity of whatever is given in the question.
so if the question tells me a ball is falling vertically down with a speed of 2, we assume that speed is the final and the initial is 0?
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4 weeks ago
#6
(Original post by interlanken-fall)
so if the question tells me a ball is falling vertically down with a speed of 2, we assume that speed is the final and the initial is 0?
That doesn't make a lot of sense in isolation. At what point does it have the speed of 2? Since gravity is always acting, speed is not constant throughout the motion
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#7
(Original post by davros)
That doesn't make a lot of sense in isolation. At what point does it have the speed of 2? Since gravity is always acting, speed is not constant throughout the motion
a small stone mass 400g projected vertically upwards with speed 10ms and it experiences resistance of N
Last edited by interlanken-fall; 4 weeks ago
0
4 weeks ago
#8
(Original post by interlanken-fall)
a small stone mass 400g projected vertically upwards with speed 10ms and it experiences resistance of N
OK, so in this case if you choose upwards as the positive direction then you would take u = +10. Not sure where your "speed of 2" has come from in the previous post
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