Need Physics help to to solve a problem about a block at rest and the forces acting on it

Physics
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A 15.0-lb block rests on the floor. (a) What force does the floor exert on the block? (b) A rope is tied to the block and is run vertically over a pulley. The other ends is attached to a free-hanging 10.0-lb object. What now is the force exerted by the floor on the 15.0-lb block? (c) If the 10.0-lb object in part (b) is replaced with a 20.0-lb object, what is the force exerted by the floor on the 15.0-lb block?

Dec 10th, 2015

Thank you for the opportunity to help you with your question!

(a) The bloc is at rest ===> the net force on it is zero. The downward force of gravity (mg) is balanced by the upward force the floor exerts on the block. 
Weight of the block=mg=>15lb=6.8 kg

Weight =6.8*9.81N=66.71N

b) The downward force of gravity (mg) is balance by two upward forces: the tension in the rope and the force of the floor on the 15 lbs block. 

F=m1g-m2g,m1=6.8 kg and m2=4.5 kg

 F=(6.8-4.5)*9.81N=22.6N

c)The upward force is 0,as the block is lifted vertically because m2>m1 ,or the tension in the rope is greater causing the block to move upward with some acceleration.


I hope this would help you,but if you have any doubt regarding this,you can surely ask. :)
Dec 10th, 2015

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Dec 10th, 2015
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