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fayled
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I seem to be getting into a bit of a mess with my signs when using Newton's second law in classical mechanics. Here's an example (I am fine with completing the question, it's just when I look at alternative ways of setting up axes and solving it, things change):
A ball of mass m is projected vertically upward at velocity vo. The ball experiences an air resistance force (in addition to gravity) of the form -αv2 where α>0 is constant and v is the velocity, and reaches a maximum height h before it returns back to the point of projection.
Write down the equations of motion of the ball during its upward and downward journeys.
Obviously the question doesn't end there but this is the only relevant part.
F=md2x/dt2
Ok, so when we're going up, let's say I take my x-axis going upwards. Then I have a particle of mass m, with mg and αv2 as the downward forces. So I write -αv2-mg=md2x/dt2 and solve, which works for the remainder of the question.
However what if I say my x-axis points downwards now. Then I have my forces down too, i.e in the x direction, so these would be positive. So I write αv2+mg=md2x/dt2 and these give different solutions. Clearly I'm missing something fundamental here (although I'm sure I've always worked in this way and not had problems) - making the RHS negative obviously works but I can't see why I'd do that (besides the acceleration is clearly downwards anyway so wouldn't that mean it would be negative in the first instance and positive now?).
Likewise going down, x-axis upwards, I have αv2 going up, mg going down, so I write αv2-mg=md2x/dt2, which gives the correct results. Then x-axis downwards, forces still in the same direction so I have mg-αv2=md2x/dt2 which again gives a different solution to above.
Can anybody explain what is wrong, thanks :)
Homework Statement
A ball of mass m is projected vertically upward at velocity vo. The ball experiences an air resistance force (in addition to gravity) of the form -αv2 where α>0 is constant and v is the velocity, and reaches a maximum height h before it returns back to the point of projection.
Write down the equations of motion of the ball during its upward and downward journeys.
Obviously the question doesn't end there but this is the only relevant part.
Homework Equations
F=md2x/dt2
The Attempt at a Solution
Ok, so when we're going up, let's say I take my x-axis going upwards. Then I have a particle of mass m, with mg and αv2 as the downward forces. So I write -αv2-mg=md2x/dt2 and solve, which works for the remainder of the question.
However what if I say my x-axis points downwards now. Then I have my forces down too, i.e in the x direction, so these would be positive. So I write αv2+mg=md2x/dt2 and these give different solutions. Clearly I'm missing something fundamental here (although I'm sure I've always worked in this way and not had problems) - making the RHS negative obviously works but I can't see why I'd do that (besides the acceleration is clearly downwards anyway so wouldn't that mean it would be negative in the first instance and positive now?).
Likewise going down, x-axis upwards, I have αv2 going up, mg going down, so I write αv2-mg=md2x/dt2, which gives the correct results. Then x-axis downwards, forces still in the same direction so I have mg-αv2=md2x/dt2 which again gives a different solution to above.
Can anybody explain what is wrong, thanks :)
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