WAEC Physics 2017 Theory — Question 13
Question 13 of 33 from the West African Examinations Council (WAEC) Physics 2017 Theory paper, with the correct answer and a full explanation.
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8(e) A body is released from rest at the top of a plane inclined at 30° to the horizontal and 4.0m high. If the coefficient of friction between the body and the plane is 0.3, calculate the time the body takes to reach the bottom of the plane
Model answer
F_net=ma. mg sinθ - Fr = ma. mg sinθ - μmg cosθ = ma. Dividing through by m: g sinθ - μg cosθ = a. a = 10xsin30 - 0.3x10xcos30 = 5-2.6 = 2.4ms^-2. Distance D covered by the body: sin30=4/D ∴ D=4/sin30=8m. From s=ut+1/2at²; s=D=8m, u=0 (at rest), a=2.4m/s: 8=1/2x2.4xt² → 16/2.4=t² → t²=6.67 ∴ t=√6.67=2.58s.
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