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

[Sculter]

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?

 

[Doc Al]

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.

 

[Sculter]

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
-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.

 

[Doc Al]

*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

-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.

 

[Sculter]

oh, great now i know why, i didn't realize it, thank you
but how do i find the angle?

 

[Doc Al]

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.)

 

[Sculter]

Thanky you very much!
Your really helped me