One block of 2.0 kg mass is placed on top of another block of 3.0 kg mass. The coefficient of static friction between the two blocks is 0.2. The bottom block is pulled with a horizontal force F such that both the blocks move together without slipping. Taking acceleration due to gravity as 10 m/s², the maximum value of the frictional force is :
- A.50 N
- B.30 N
- C.4 N
- D.10 N
Correct answer
Explanation
The correct answer is C, 4 N. The friction between the blocks is the only force that drags the top block along, and its largest possible value is μ times the normal force from the top block.
The top block of 2.0 kg presses down on the lower block with its weight, so the normal force is mg = 2.0 × 10 = 20 N. Limiting static friction is μN = 0.2 × 20 = 4 N. This 4 N gives the top block a maximum acceleration of 4 ÷ 2 = 2 m/s², so the two blocks move together only while F stays at or below 5 kg × 2 m/s² = 10 N. Beyond that, the top block starts to slip.
A is wrong because 50 N is the combined weight of both blocks, not a friction force. B is wrong because 30 N is the weight of the lower block alone. D is wrong because 10 N is the largest pulling force F that keeps the blocks together, not the friction between them.
Exam tip: limiting friction = μN; for stacked blocks, N comes only from the block on top.
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