Block of Sandpaper along the ceiling

[indietro]

Homework Statement

A 5.49 kg block covered in sandpaper is pushed along the ceiling of a room under construction. The block is pushed across the ceiling with a force of 81.6 N directed at an angle of 70.0° to the horizontal. If the coefficient of kinetic friction between the paper and the ceiling is 0.533, what acceleration does the block undergo?

Homework Equations

component forces in x-direction = Fextcos70 - Ff

The Attempt at a Solution

ma = Fextcos70 - uFn
= 81.6cos70 - (0.533)(5.49*9.8)
= 27.9 - 28.7

** so i would get a negative number: that doesn't really make sense. Here I say that the normal force is equal in magnitude to the weight. Is this correct? Or do i need to include the y-component of the push when looking for the normal force?
Help would greatly be appreciated!

 

[chris097]

I'm having trouble with the same problem

 

[willem2]

Here I say that the normal force is equal in magnitude to the weight. Is this correct? Or do i need to include the y-component of the push when looking for the normal force?

you certainly need to do that. suppose you get an hydraulick and push the block against the
ceiling with 10000N. do you think the normal force is still m.g

It is possible to end up with a negative number. That just means that the friction is too large
and the block won't move

 

[indietro]

Thank you! when i also consider the y-component of the push when finding Fn it works out :) and ends up with a positive acceleration