The diagram shows a fixed block with a horizontal top surface and a surface which is inclined at an angle of θ° to the horizontal, where sin θ = 3/5. A particle A of mass 0.3 kg rests on the horizontal surface and is attached to one end of a light inextensible string. The string passes over a small smooth pulley P fixed at the edge of the block. The other end of the string is attached to a particle B of mass 1.5 kg which rests on the sloping surface of the block. The system is released from rest with the string taut. [Figure X.X]
- (i)Given that the block is smooth, find the acceleration of particle A and the tension in the string.[5]
- (ii)It is given instead that the block is rough. The coefficient of friction between A and the block is μ and the coefficient of friction between B and the block is also μ. In the first 3 seconds of the motion, A does not reach P and B does not reach the bottom of the sloping surface. The speed of the particles after 3 s is 5 m s⁻¹. Find the acceleration of particle A and the value of μ.[9]
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Mark scheme
Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.
- M1 Apply Newton's second law to A or to B or to the system
- A1 Any two correct equations
- M1 Solve 2 simultaneous equations for a and/or T or use the system equation.
- A1 a = 9/1.8 = 5 ms⁻²
- A1 T = 1.5 N
- M1 v = u + at used with t = 3, u = 0, v = 5
- A1 a = 5/3 = 1.67
- B1 For either reaction
- M1 Use F = μR for either term
- M1 Apply Newton's second law to A or to B or to the system
- A2/1/0 A1 Correct equation for A or B A2 Correct equations for A and B OR A2 Correct system equation
- M1 Solve for μ from equations with correct number of terms
- A1 μ = 0.4 = 2/5
- B1 Find distance travelled in 3 secs
- B1 PE loss =
- B1 KE gain =
- M1 Use Work/Energy equation
- A1 μ = 2/5 = 0.4
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