8 An elastic string has natural length 1.5 metres and modulus of elasticity 120 newtons. One end of the string is attached to a fixed point, \(A\), on a rough plane inclined at \(20 ^ { \circ }\) to the horizontal. The other end of the elastic string is attached to a particle of mass 4 kg . The coefficient of friction between the particle and the plane is 0.8 .
The three points, \(A , B\) and \(C\), lie on a line of greatest slope.
The point \(C\) is \(x\) metres from \(A\), as shown in the diagram. The particle is released from rest at \(C\) and moves up the plane.
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- Show that, as the particle moves up the plane, the frictional force acting on the particle is 29.5 N , correct to three significant figures.
- The particle comes to rest for an instant at \(B\), which is 2 metres from \(A\).
The particle then starts to move back towards \(A\).
- Find \(x\).
- Find the acceleration of the particle as it starts to move back towards \(A\).
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