AS CIE Physics Paper 1 Questions on Newton's Laws and Pressure

[farahroslend]

Hi
Explanation required for QUESTION 7 AND 11, from past years :
http://papers.xtremepapers.com/CIE/...and AS Level/Physics (9702)/9702_s09_qp_1.pdf

Can anyone please explain why the answer is A (and not C or D) for question 7. and
why the answer is A (and not C) for question 11?

Thank you for your help.

 

[h4rriet]

Hi
Explanation required for QUESTION 7 AND 11, from past years :
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s09_qp_1.pdf

Can anyone please explain why the answer is A (and not C or D) for question 7. and
why the answer is A (and not C) for question 11?

Thank you for your help.

7. A consequence is something that happens because of something. C says that 'conservation of energy is a consequence of the 3rd law'. Which means that energy is conserved because of the 3rd law, in other words, energy is conserved because every action has an equal but opposite reaction. That isn't true; energy is not conserved because of that. It's conserved because the total energy of the universe is constant. D says that 'conservation of linear momentum is a consequence of the first law'. The first law says that a moving object keeps moving at a constant speed, and a stationary object remains stationary, unless an external force acts. That's the same as saying that the momentum of an object stays constant, unless an external force acts on it. So the conservation of linear momentum = the first law.
11. Upthrust is a force caused by a difference in pressure. There's a pressure difference between the top and the bottom of the sphere (because pressure increases with depth). That's why the two forces can't be the same, because they're at different depths. The forces at S and Q are the same because the depth of the points is the same.