A 4.0 kg block is on a horizontal table. A horizontal force of 20 N is applied to the block, but the block does not move. What is the magnitude of the static friction force acting on the block?
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Correct answer
B. 20 N
Principle or equation
Newton's first law: if an object is at rest and remains at rest, the net force on it must be zero. Therefore, the static friction force must exactly balance the applied horizontal force.
Why this answer is correct
The block is at rest, so the horizontal forces must sum to zero. The applied force is 20 N to the right, so the static friction force must be 20 N to the left. Its magnitude is 20 N.
Example
If a 10 N force is applied and the block remains at rest, static friction is 10 N.
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