How to find forces acting on object on a slope?

  • Thread starter urbano
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  • #1
urbano
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Homework Statement




an object is at rest on a slope at angle of σ

Using trigonometry, solve the forces acting at Ft and Fn

2ezlnqa.jpg




Homework Equations



Soh
Cah
Toa



The Attempt at a Solution



sin σ = G/Ft
sin σ /Ft = G
G/ sin σ = Ft (my final answer)

Book says correct answer is Ft = G sinσ


Fn = ??? I wanst even sure how to strat this using σ

The book says the correct answer is Fn = G cosσ

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
NascentOxygen
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The force G can be resolved into 2 components: one which is normal to the slope (the exact opposite to Fn), and one which is parallel to the slope (the exact opposite to Ft).

Draw the triangle of forces showing how G equals the vector sum of these two forces.
 
  • #3
urbano
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edited...I'll be back
 
Last edited:
  • #4
urbano
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Thanks NascentOxygen. Unfortunately I dont quite get why the answers are what they are. I have drawn myself a new triangle which shows the two vector forces but I still don't understand where the answers have come from.

Especially Fn = G cosσ. I thought cos in basic terms meant adjacent/hypothenuse . So if the sloped line is the hypothenuse and the bottom line is the adjacent once I draw the line which is exact opposite to Fn I now no longer have a right angle triangle.....arghh!! I'm just so confused...
 
  • #5
NascentOxygen
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Show your diagrams.
 
  • #6
urbano
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here is my latest diagram
2yw8ygz.jpg


I'm now starting to think Soh Cah Toa isn't actually relevant there ?

Fn = G cosσ Ft = Gσ

I'm unsure where these two formulas/equations have come from and why Soh Cah Toa isn't relevant here...

I must be missing something fundamental
 
  • #7
NascentOxygen
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Science Advisor
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here is my latest diagram
It's the triangle beneath the object which you need to focus on.
redraw it separately, and larger, and label what you know. Its vertical line represents G Newtons.
 
  • #8
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
here is my latest diagram
It's the triangle beneath the object which you need to focus on. Redraw it separately, and larger, and label what you know. Its vertical line represents G Newtons.
 

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