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Force on a Box - Find Weight

  • Thread starter erok81
  • Start date
  • #1
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Homework Statement



You push down on a box at an angle 15° below the horizontal with a force of 650 N. If the box is on a horizontal surface and the coefficient of static friction is 0.7, what is the heaviest box you will be able to move?

Homework Equations



F=ma

The Attempt at a Solution



So I've solved this problem using Fnet which was 650cos15 for the horizontal force and then subtracted [tex](\mu_s)(9.80)(m)[/tex] The ma part goes to zero since it's not accelerating.

Therefore I get [tex]650cos15 = (\mu)(9.80)(m)[/tex] and then solve for m.

I thought that was it. But now that I thinking about it more, there is a vertical component for the force. I'd imagine that extra force would make the box seem to weigh more, thereby increasing friction, making the box weigh less.

Do I factor that vertical force in? If so, how?
 

Answers and Replies

  • #2
464
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Ah! Maybe like this?

[tex]650cos15 = [((\mu)(9.80)(m))+(650sin15)[/tex]

All I did here was add in the vertical force component to the friction.

Yeah...that isn't right since my box ends up weighing 2kg.
 
Last edited:
  • #3
Your above equation is correct.Though you will multiply the coefficient of friction to both the weight as well as 650sin15
 
  • #4
464
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Nice.

So something like this?


[tex]650cos15 = [((\mu)(9.80)(m))+((\mu)(650sin15))][/tex]
 
  • #5
Yeah,that's correct.
 
  • #6
464
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Thanks for the help!:approve:
 

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