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Calculating the frictional forces on a sliding box on a ramp
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[QUOTE="kuruman, post: 5984303, member: 192687"] There seems to be a confusion of negative signs regarding magnitudes and directions of vectors especially when you substitute numerical values. The problem gives you the two accelerations as negative numbers. We know that the acceleration is downhill regardless of the direction of the velocity therefore the positive axis is uphill and the negative direction is downhill. Now when the block is moving uphill, all vectors point downhill, so the proper equation to write is (and I put numbers although I find this distasteful, but I have to make the point clear) $$0.123(kg)*[-9.81(m/s^2) \sin24^o ]+(-R_{up})=0.123(kg)*[-6.9(m/s^2)]$$ For the downhill motion, the force of friction changes sign and $$0.123(kg)*[-9.81(m/s^2) \sin24^o ]+R_{down}=0.123(kg)*[-1.16(m/s^2)]$$ Note that ##R_{up}## and ##R_{down}## denote, respectively, the [B]magnitude [/B]of the frictional force when the block is sliding up and down the incline. As such, when the equations are solved, they should come out positive. [/QUOTE]
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Calculating the frictional forces on a sliding box on a ramp
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