About the angle between force and displacment, W=F.d cosƟ

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W=F.d cos Ɵ, in a problem Ɵ=30, but in the solution he used Ɵ=60, and vice versa in other problems, can anyone explain why?
 
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  • #2
Doc Al
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Is θ the angle between F and d, or is it something else? Please post the full problem so we know what you're talking about.
 
  • #3
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Is θ the angle between F and d, or is it something else? Please post the full problem so we know what you're talking about.
problem like:

*calculate the force acting on an object if the work done to move it a distance 50m equal 2500J and if the angle between the force and the perpendicular to motion is 30
the answer: w=fd cosθ , F=2500/50xcos60 = 100N

*a man of mass 70kg. goes upstaris of lenth 50m. calculate the work done if g=10m/s2 http://im88.gulfup.com/rmDnTK.png [Broken]
-answer W=Fd cosθ, 70x10x50 x cos30

-problem with no changing in angles-

*a force of 100N acts on an object and moves it a distance 2.5m, calculate the work by the force if the force is inclined by an angle 60 on the direction of motion
answer: W=100x2.5x cos60= 125J

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i don't know why he changes angles and when, i think i'm missing something.
 
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  • #4
Doc Al
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*calculate the force acting on an object if the work done to move it a distance 50m equal 2500J and if the angle between the force and the perpendicular to motion is 30
the answer: w=fd cosθ , F=2500/50xcos60 = 100N
Note that the angle given is not between F and d, but between F and the perpendicular to d. It's up to you to figure out the correct angle!

*a man of mass 70kg. goes upstaris of lenth 50m. calculate the work done if g=10m/s2
proxy.php?image=http%3A%2F%2Fim88.gulfup.com%2FrmDnTK.png

-answer W=Fd cosθ, 70x10x50 x cos30
Again, the angle given is the angle of the stairs with the horizontal. What you need is the angle between the force (gravity) and the displacement.

-problem with no changing in angles-

*a force of 100N acts on an object and moves it a distance 2.5m, calculate the work by the force if the force is inclined by an angle 60 on the direction of motion
answer: W=100x2.5x cos60= 125J
Here they just give you the required angle, so you can just plug it in.

i don't know why he changes angles and when, i think i'm missing something.
To use the work formula you need the angle between the force and the displacement. Sometimes you'll be given that angle directly; other times you'll have to figure out the correct angle to use.
 
  • #5
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oh, great now i know why, i didn't realize it, thank you
but how do i find the angle?
 
  • #6
Doc Al
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oh, great now i know why, i didn't realize it, thank you
You are welcome.

but how do i find the angle?
Start by drawing yourself a diagram. And use the properties of right triangles. (For example, the two angles of a right triangle are complementary. They add to 90 degrees.)
 
  • #7
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Thanky you very much!
Your really helped me
 

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