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# M3 Newton's law of gravitation question

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1. In the following questionthe partucle is moving towards the centre of the earth which means the same direction as the acceleration. I took it that this means that F = (GmM)/(x^2) instead of F = (-GmM)/(x^2) is my assumption wrong? The correct answer as given in the book only comes if F = (-GmM)/(x^2). I attached my working.
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2. (Original post by thebrahmabull)
In the following questionthe partucle is moving towards the centre of the earth which means the same direction as the acceleration. I took it that this means that F = (GmM)/(x^2) instead of F = (-GmM)/(x^2) is my assumption wrong? The correct answer as given in the book only comes if F = (-GmM)/(x^2). I attached my working.
I had this exact problem about a month ago. After asking Zacken , I think he said something on the lines of the reason why the force/acceleration is negative is because the displacement x from the earth is decreasing as the particle is in motion. I still struggle to get my head around this but I guess you could always use common sense, e.g if the speed at the surface isn't greater than the original speed then you know you have ****ed up somewhere
3. (Original post by TheFarmerLad)
I had this exact problem about a month ago. After asking Zacken , I think he said something on the lines of the reason why the force/acceleration is negative is because the displacement x from the earth is decreasing as the particle is in motion. I still struggle to get my head around this but I guess you could always use common sense, e.g if the speed at the surface isn't greater than the original speed then you know you have ****ed up somewhere
I see. Thanks!
4. You need to resolve in the direction of increasing, basically.

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