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# Couple of questions AQA Physics 4 need little help with if possible! watch

1. Hi there I have attached both problems. The correct answer is circled red from the mark scheme. I can't work out how it gets to it!!
Hope someone can help!!
Attached Images

2. For the first question you have to be careful. You know that g=GM/r^2, it is proportional to M/r^2. You know that volume is proportional to r^3 (V=4/3pir^3) Hence if the diameter increases by 100x, the radius also does, and the volume increases by 10^6. To keep the density constant the mass must also increase by 10^6 since density=mass/vol. Now since g is proportional to M/r^2. r has increased by 100^2 and M has increased by 10^6. Therefore g must have increased by 100x. Therefore g on the surface of the star is 100x8=800NKg^-1.
3. For the second question, weight is proportional to the gravitational force on that object. Hence if R increases to 2R then the force decreases by 4, but since the mass increases by two, this increases the force on the mass by a factor of 2. 2x1/4 = 1/2 therefore the weight halves to W/2.

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Updated: January 13, 2015
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