Box Pushed Up an Incline Ramp: Basic Newton's Law Questions

  • #1
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Homework Statement


A 90 kg box is pushed by a horizontal force F at constant speed up a ramp inclined at 28°, as shown. Determine the magnitude of the applied force.
  1. when the ramp is frictionless.
  2. when the coefficient of kinetic friction is 0.18
upload_2015-7-22_11-43-55.png

Homework Equations


F=mg
FN=mg
μ= Fk/FN

The Attempt at a Solution


a) mg=(90)((.8)= 882N=Fg

sin28=Fgx/882N
(0.469)(882)= Fgx
414N=Fgx= magnitude of applied force

b) FN=mg= 882N
μ=Fk/FN
0.18=Fk/882
(882)(0.18)= Fk
Fk= 158.76

** This is where I am unsure of my approach**

Fapplied= Fgx+Fk
=414N+158.76 N
= 572N

That seems like too much to me, but I'm not quite sure. Does it look like I have that equation correct ,or am I supposed to subtract the value I found for Fk from FgX, or something totally different. Any help would be very much appreciated
 

Answers and Replies

  • #2
501
66
FN=mg= 882N
μ=Fk/FN
0.18=Fk/882
(882)(0.18)= Fk
Fk= 158.76

You have not drawn FN correctly . Redraw correctly and try again .

Hope this helps .
 
  • #3
4
0
You have not drawn FN correctly . Redraw correctly and try again .

Hope this helps .
oh okay, is it opposing the Fy aspect instead of Fg? I was confused when I was drawing that! Thank you for your response
 
  • #4
SammyS
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oh okay, is it opposing the Fy aspect instead of Fg? I was confused when I was drawing that! Thank you for your response
Yes. The normal force opposes that component of gravity which would cause acceleration perpendicular to the surface ( the incline plane).
 

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