  Question

A block of mass 1 kg lies on a hotrizontal surface in a truck. The coefficient of static friction between the block and the surface is 0.6. If the acceleration of the truck is $$5 m/sec^2$$, find out the frictional force acting on the block.

A
10N  B
6N  C
4N  D
5N  Solution

The correct option is D 5NIn these type of question we assume first there is no relative motion between surfaces and find out  friction co-efficient. If this friction co-efficient is less than static friction co-efficient then our assumption is right. Otherwise we solve using static friction Co-efficient.Normal force due to truck on block (N) = mg = 1$$\times$$ 10 = 10 NewtonBlock will move due to friction force Friction force = $$\mu N$$ = $$10\mu$$Assuming no sliding between surfaces Acceleration of block (a)= acceleration of truck = 5 $$m{ s }^{ -2 }$$Writing force equation,$$10\mu$$ = ma$$10\mu$$ = 1$$\times$$5$$\mu$$ = 0.5 which is less than 0.6(static friction co-efficient), so our assumption was correct. So friction force = $$\mu \times N$$                         = 0.5 $$\times$$10 = 5 NewtonPhysics

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