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a)$47 \textrm{N}$
b)$17.0 \textrm{N}$
c)$0 \textrm{N}$

Giancoli 6th Edition, Chapter 4, Problem 19


Chapter 4, Problem 19 is solved.

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Transcript for this Giancoli solution

This block will experience gravity downwards of course and that’s equals seventy seven Newtons in the 6th Edition. It will also experience the force of the hanging weight that is going to be pulling it upwards ‘Fw’ and then also there will be the normal force exerted by the floor upwards ‘FN’ and since this block is at rest it means that the total up force equals the down force. In other words the net force is zero. And we can say: FN plus Fw equals the Fg So in part a solving for the normal force it’s is Fg minus Fw and normal force is seventy seven newtons minus the thirty newtons exerted upwards by the hanging weight, that’s a total of forty seven Newtons left over for the normal force. Part b: normal force is seventy seven newtons minus sixty newtons giving us seventeen newtons as normal force in this case. Then part c they are trying to trick you and you have to realize that the normal force will be zero. You can’t blindly use this formula you have to think about the physics behind it and realize that if the hanging weight has a force that exceeds gravity then this block will no longer be even touching the table and therefore the surface can’t exert any force on it at all. The normal force is zero in part c. In the 5th Edition the numbers are slightly different, it’s a seventy newton block and so we’ll just replace seventy seven with seventy and we get our 5th Edition answers are forty point zero newtons and ten point zero newtons and the answer for part c is still zero.