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Three blocks A, B and C of masses 4 kg, 2 kg and 1 kg respectively, are in contact on a frictionless surface, as shown. If a force of 14 N is applied on the 4 kg block then the contact force between A and B is:

1. 6 N
2. 8 N
3. 18 N
4. 2 N

Correct answer: 6 N

Solution

The total mass of the system is 4 + 2 + 1 = 7 kg. The acceleration of the system is a = F/m = 14/7 = 2 m/s². The contact force between A and B is due to the acceleration of blocks B and C, which have a combined mass of 3 kg. Thus, the contact force is F = ma = 3 × 2 = 6 N.

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