Finding acceleration of an object (with friction)

  • Thread starter Joe26
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  • #1
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Homework Statement


A 1,300-N crate is being pulled across a level floor at a constant speed by a force of 310 N at an angle of 20.0° above the horizontal. If coefficient of kinetic friction is 0.207, find the acceleration of the crate.


Homework Equations


Fk= (uk)(N)
F=ma


The Attempt at a Solution


Fx= 310cos20
Fy= 310sin20

N=1300-310sin20

Fk=(uk)(N) = (0.207)(1300-310sin20) = 247.15 N

sumFx= Fx-Fk =310cos20-247.15 = 44.15 N

F=ma=44.15
a=44.15/1300 =0.0340 m/s^2

Answer says a=0.331

Can someone point out what I did wrong? Thanks
 
Last edited:

Answers and Replies

  • #2
SammyS
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Doesn't gravity exert a force in the y direction, contributing to Fy ?

The Fy you have is downward. Do you want that to be negative ?
 
  • #3
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im not sure if I quite follow. Yes, force of gravity (1300N) is exerted in the y-direction:

F1y= 310sin20
N=1300-310sin20
sumFy=N+F1y-1300N=0

I'm not sure what is wrong. Can someone help me out?
 
  • #4
Doc Al
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The applied force is downward:
a force of 310 N at an angle of 20.0° below the horizontal.
 
  • #5
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sorry that should be above the horizontal
 
  • #6
Doc Al
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F=ma=44.15
a=44.15/1300 =0.0340 m/s^2
1300 N is the weight of the crate, not its mass.
 
  • #7
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so, how would i be able to find the acceleration?
 
  • #8
Doc Al
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so, how would i be able to find the acceleration?
Just like you tried to do, only first find the mass of the crate. How do weight and mass relate to each other?
 
  • #9
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Got it, thanks. W=mg...find m using weight and use f=ma to find acceleration.
 

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