Question:
in part 19 of laws of motion class 11; problem 03- why is the force applied on the body is considered as the maximum value of static friction?
Answer:
There are two types of friction: Static friction and Kinetic friction
Static friction: The static friction is a force which keeps the body at rest. If the object should be moved, then it has to overcome the static friction.
If a force F is applied to move the object at rest, then the static friction is equal and opposite to the force F applied.
Kinetic friction: A moving object experiences kinetic friction.
The problem states: A rubber ball is pressed against a wall by a spring. The elastic force of the spring is normal to the wall and is equal to 200N. Which force should be applied to the bar to initiate motion? Co-efficient of friction for rubber over concrete is 0.75.
The word initiate motion indicates maximum value of static function should be used. Hence, Force applied Fa = Maximum static friction = µs * N = 0.75 * 200 = 150N
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