The figure shows a brick being pulled across a floor using a spring balance.  The spring balance reading at the instant the brick JUST begins to move gives a measure of:
AAtmospheric pressure on the brick
BThe sliding friction
CThe static friction (limiting friction)
DThe weight of the brick
Answer & Solution
Correct answer: C. The static friction (limiting friction)
The brick stays at rest until the applied pull just overcomes static friction. The spring-balance reading at that 'about to move' instant therefore measures the maximum static (limiting) friction between the brick and the floor.
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