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H2 Physics 2019 Paper 2 · Demo Student · read-only sample
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Student
Hi Cherlock — for Q1(c) I got T = 185 N but the marking says it should be around 280 N. I balanced the horizontal components of the wire and the cable. Why is that wrong?
Cherlock
Good question. Horizontal force balance only tells you the cable and wire horizontal components are equal — it does not give the cable tension on its own, because the base of the pole can still push horizontally.
For this part you need moments about the base of the pole. Pivot there so the ground reaction has zero moment, then equate the moment of the wire to the moment of the support cable. That isolates T without needing the reaction.
For this part you need moments about the base of the pole. Pivot there so the ground reaction has zero moment, then equate the moment of the wire to the moment of the support cable. That isolates T without needing the reaction.
Student
So the 150 N in the question is the wire tension, not my answer for the cable?
Cherlock
Exactly. 150 N is the given wire tension. Your job is the support-cable tension T. With moments about the base you should get roughly 277–280 N (2 s.f.), not 185 N.
Student
One more — Q4(a)(ii). I used 9.11 × 10⁻³¹ kg for the mass. The feedback says that is wrong even though my method looked fine.
Cherlock
Your energy method is fine: convert 64 eV to joules, then v = √(2KE/m), then E = Bv. The particle is a proton, so use m ≈ 1.67 × 10⁻²⁷ kg, not the electron mass. With the proton mass you get v ≈ 1.11 × 10⁵ m s⁻¹ and E ≈ 4980 N C⁻¹.
Student
Got it — wrong particle mass cascading into E. Thanks!
Cherlock
That is a classic Paper 2 trap. Next time: check which particle the stem names before picking m from the data list. You are in good shape on Q1(a)/(b) and most of Q3 — prioritise moments, Malus (cos²), and particle identity.
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