Free-body Diagram Problem

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


A dog pulls a sled with unspecified force at 25° above horizontal. Mass of sled-passenger-rope partile is 80 kg and there is negligible friction. Find the maximum magnitude of force F that can be applied to the rope without lifting the sled off the surface.

Homework Equations


sum of forces = ma
normal force is Fn
mass of particle is m

The Attempt at a Solution



Fxnet = Fcos(25) = max
Fynet = Fsin(25) - 9.8*m + Fn

In any case where the sled is not lifted, Fynet = 0, since acceleration is in direction of x only (for this problem).

However, I am confused about how to find maximum force before sled is lifted.
 

Answers and Replies

  • #2
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When you start to lift the sled you have: Fsin(25)= 9.8*m. In that moment, you have no Fn because the sled is not touching the ground.
 
  • #3
671
2
Don't think of being lifted off the ground in terms of the net force, but in terms of the normal. When the normal is 0 or negative, then you have lift-off.

Though the two approaches are completely equivalent, I find that, personally, the latter is simpler to follow.
 

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