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do u have any specific situation?
well in general, u solve it like any other force related problem. draw a force-body diagram. resolve all the forces acting on the body in 2 axes. then calculate the net force acting on it. here the important thing to keep in mind, is the direction of the frictional force. it will always be in the opposite direction to the relative motion between the 2 surfaces in contact.
hii migo !! methinks you should have some problem to discuss upon. The direction of friction force is not always opposite to direction of motion. Friction sometimes acts along the direction of motion while sometimes opposite . You'd understand it's real nature when you solve problems !!
yes, as @meera_yadav pointed out, it does not always oppose the direction of motion. as I said earlier, it always opposes the 'relative motion'. u need to understand the difference between these two terms.
remember vaidelhi says " it will always be in the opposite direction to the RELATIVE motion between the 2 surfaces in contact."