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April 16, 2014

Homework Help: Physics/Math

Posted by COFFEE on Saturday, February 10, 2007 at 5:56pm.

A 3.0 kg block, initially in motion, is pushed along a horizontal floor by a force F of magnitude 18 N at an angle = 45 with the horizontal. The coefficient of kinetic friction between the block and floor is 0.25. (Assume the positive direction is to the right.) Calculate the magnitude of the frictional force on the block from the floor. Calculate the magnitude of the block's acceleration.


Break the force F in to vertical and horizontal components.

Friction= (mg + force vertical)mu

net force= ma
horizontal F - force friction = ma

Ok, so this is what I tried...

T = uk*m*g / ((cos theta)+uk(sin theta))
T = ((.25)(3)(9.8)) / ((cos -45)+(.25)(sin -45))
T = 13.9 N

then,

FNet = m*g - T*sin(theta)
FNet = (3)(9.8) - (13.9)*(sin -45)
FNet = 39.2

then,

fk = uk*FNet
fk = .25*39.2
fk = 9.8

Well that was the wrong answer but I don't see anything wrong with my approach. Any suggestions???

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