Two particles P and Q, of mass 2 kg and 3 kg respectively, are joined by a light inextensible string. Initially the particles are at rest on a rough horizontal plane with the string taut. A constant force F of magnitude 30 N is applied to Q in the direction PQ, as shown in Figure 4. The force is applied for 3 s and during this time Q travels a distance of 6 m. The coefficient of friction between each particle and the plane is µ .
Find
(a) the acceleration of Q - 4/3
(b) the value of µ - 10/21
(c) the tension in the string - 320 N
When the particles have moved for 3 s, the force F is removed.
(e) Find the time between the instant that the force is removed and the instant that Q comes to rest.
For part e) So I get that v would be 4ms and you use that as u and for
deceleration you resolve but I don't understand why the tension is removed and not modelled when considering the motion of Q and also can anyone tell me how to know which particles to consider each time when resolving?