Lynn Gweeny 3 Report post Posted October 25, 2017 Hello! Could somebody explain the answers to the MCQs below please? Thanks! Share this post Link to post Share on other sites

kw0573 1,202 Report post Posted October 25, 2017 4. (Vectors not drawn to scale). Fnet = ma = mgsin θ - Ff, where positive is positive x. Ff = mg sin θ - ma 17. The main idea is to use sub topic 5.3 equation ε = I(R + r), ε is emf, R is circuit resistance, r is internal resistance. To use this equation, we have to simplify the circuit into just 1 equivalent battery source and 1 equivalent load/resistor. We already have just 1 resistor but we need to combine the 2 cells. Equivalent resistance of cells in parallel is 1/R = 1/R_{1} + 1/R_{2} ==> r = 1Ω. Equivalent voltage of cells in parallel is V = V_{1 }= V_{2} = ε = 1.6 V. The circuit is equivalent to a 1.6V cell of 1Ω connected to a 3Ω resistor. 1.6V = I (3Ω + 1Ω) ==> I = 0.4A. 18. This is just math. Slope is rise/run = -0.6/0.2 = -3 . log B = -3 log x + const ==> B = 10^{const }x^{-3}, B is proportional to x^{-3}. 19. This question shows up often. By practicing more past papers you should become familiar with most of these problems. Refer to 1st equation under subtopic 6.2 A is incorrect: No gravitational forces only if infinitely far away from Earth. B is correct: a = F/m and by same equation, acceleration is dependent upon distance from Earth, and mass of Earth, not dependent upon mass of object. C is incorrect: equilibrium, in IB, refers to translational equilibrium, which means a balance of forces. As the astronaut and the capsule are orbiting (circular motion), the net force is non-zero. D is incorrect: F is dependent upon mass of object, so since astronaut and capsule have different mass, the force acted upon them is different. 20. c = fλ (subtopic 4.1). E = hf (subtopic 7.1). Note c is speed of light, h is Plank's constant. When combined this give you λ = hc/E (also 7.1). Hence when energy decrease by a factor of 5 (from 20 to 4), wavelength increase by a factor of 5, namely from 1* 10^{-7} to 5 * 10^{-7} 24. P = eσ AT^{4} (subtopic 8.2). Substituting T = 1.02T_{1} to get T^{4} = 1.082 ... T_{1} So C. You don't have a calculator but you just have to do this by hand (eg 1.02 * 1.02 = 1.0404, then truncate to 1.04 * 1.04 = 1.0816). This is why to always plug in everything carefully and never just say oh 2^{4} = 16. In summary, 4, 19 are conceptual. 18, 20, 24 are just plug in numbers correctly. 17 is both 3 Share this post Link to post Share on other sites