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Suppose there is on object of 20.0kg sliding to the right. A person exert a force of 30N left on the object. The object has an acceleration of 2.5m/s^2 left. What is the magnitude and direction of kinetic friction?

Since the object is moving to the right (slowing down), I supposed the kinetic friction is to the left since it always opposes the direction of motion.

Take right to be positive.Thus, (-F) + (-30) = (20.0)(-2.5), -F = (-50) + (30), F = +(20N)

But isn't +20N mean the direction is right?

I'm a bit confused here.

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  • $\begingroup$ You started with (-F) + (-30): ie you actually defined F as the force to the left! $\endgroup$ – IanF1 Nov 17 '14 at 7:17
  • $\begingroup$ I wonder if you can simplify. What is acceleration with friction? What is acceleration without friction? Subtract and F=ma? $\endgroup$ – C. Towne Springer Nov 17 '14 at 7:25
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In your equation, you assumed (-F) is the frictional force, since it is acting towards right. Hence you got F = 20. Which effectively means -F = -20N

Or just assume frictional force is F irrespective of direction (assuming it is not known at this time). Then you will get F = -20N which means it acts towards right.

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