Q.
A 3-kg block of wood is on a level surface where μs=0.25 and μk=0.2. A force of 7 N is being applied horizontally to the block. Mark the correct statement(s) regarding this situation.
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a
If the block is initially at rest, it will remain at rest and friction force will be about 7 N.
b
If the block is initially moving, then it will continue its motion forever if the force applied is in the direction of the motion of the block.
c
If the block is initially moving and the direction of applied force is same as that of motion of block, then block moves with an acceleration of 1/3ms−2along its initial direction of motion.
d
If the block is initially moving and direction of applied force is opposite to that of initial motion of the block, then the block decelerates, comes to a stop, and starts moving in the opposite direction.
answer is A.
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Detailed Solution
If the block is at rest, then force applied has to be greater than limiting friction force for its motion to begin.fL=μsmg=0.25×3g=7.5Nfk the block is accelerated with an acceleration of a=F−fkm=13ms−2 and hence its speed is continuously increasing.If applied force is opposite to direction of motion, then block is under deceleration of a=−F+fkm=−133ms−2 and hence after some time block stops and kinetic friction vanishes but applied force continues to act.But as F
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