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# Mechanics Help! watch

1. (Original post by king of swords)
You have three forces at work...weight, reaction force and friction.
Equilibrium of forces // to the plane give you:
0 = 250gsin20 - Friction
If it then gives that the equilibrium is limiting, therefore:

F=uR, where R is the resultant force. So:

250g sin 20=u R

But how do I find R?

Thanks
2. (Original post by 2776)
If it then gives that the equilibrium is limiting, therefore:

F=uR, where R is the resultant force. So:

250g sin 20=u R

But how do I find R?

Thanks
Equilibrium perpendicular to the plane gives:
0 = 250gcos20 - R
3. (Original post by king of swords)
Equilibrium perpendicular to the plane gives:
0 = 250gcos20 - R
How do you get that?
4. (Original post by 2776)
How do you get that?
Like i said:
equilibrium perpendicular to the plane....the only forces perpendicular to the plane are R and 250gcos20...and they are in opposite directions....it is in equilibrium so those two forces are also equal.
5. (Original post by king of swords)
Like i said:
equilibrium perpendicular to the plane....the only forces perpendicular to the plane are R and 250gcos20...and they are in opposite directions....it is in equilibrium so those two forces are also equal.
I get this:

u= (250gsin20)/(250gcos20)
u=0.364 (3sf)

Do you get this as well?
6. (Original post by 2776)
I get this:

u= (250gsin20)/(250gcos20)
u=0.364 (3sf)

Do you get this as well?
yes
7. (Original post by king of swords)
yes
If a 2000N force is used to push the crate horizontally, show that the crate is in equilibrium.

Parallel: 2000cos20=250gsin20+250gcos20
Perpendicular: 250gsin20+2000gcos20

is this correct?
8. (Original post by 2776)
If a 2000N force is used to push the crate horizontally, show that the crate is in equilibrium.

Parallel: 2000cos20=250gsin20+250gcos20
Perpendicular: 250gsin20+2000gcos20

is this correct?
parallel: yes...
perpendicular...you haven't got an = sign anywhere
9. (Original post by king of swords)
parallel: yes...
perpendicular...you haven't got an = sign anywhere
perpendicular: R=250gcos20+2000sin20
10. (Original post by king of swords)
parallel: yes...
perpendicular...you haven't got an = sign anywhere
One other thing....you are making certain forces equal to others as though you already know it is already known to be in equilibrium...the idea is to prove it.
11. (Original post by 2776)
perpendicular: R=250gcos20+2000sin20
250gcos20=250gcos20+2000sin20

But then how do i prove that they are in equilibrium?
12. (Original post by 2776)
250gcos20=250gcos20+2000sin20

But then how do i prove that they are in equilibrium?
Well that equation doesn't work if you look at it (because R no longer equals 250gcos20)
13. (Original post by king of swords)
Well that equation doesn't work if you look at it (because R no longer equals 250gcos20)
2000cos20=F+250gsin20

R=250gcos20+2000sin20
14. (Original post by king of swords)
Well that equation doesn't work if you look at it (because R no longer equals 250gcos20)
I don't see how it can be in equilibrium....for perpendicular equilibrium....R must equal 1618.2N so the maximum value of F can only be 589.0N....but for parallel equilibrium F needs to be 1041.4N
15. (Original post by king of swords)
I don't see how it can be in equilibrium....for perpendicular equilibrium....R must equal 1618.2N so the maximum value of F can only be 589.0N....but for parallel equilibrium F needs to be 1041.4N
do you get that?
16. R- (250gcos20+2000sin20)=0

F+ (2000cos20-250gsin20)=0
17. (Original post by 2776)
R- (250gcos20+2000sin20)=0

F+ (2000cos20-250gsin20)=0
I reckon as a new force is added, u have to find the above 2 equations. and find f and r.
18. (Original post by 2776)
I reckon as a new force is added, u have to find the above 2 equations. and find f and r.
Yes so do I.....but one thing that stays constant is μ...because the crate and the surface are still the same materials.
19. (Original post by 2776)
I reckon as a new force is added, u have to find the above 2 equations. and find f and r.
R= 2986

F=1041
20. (Original post by 2776)
R= 2986

F=1041
And so since F=uR

We know F and we know R.

and so u=F/R

and I get u = 0.349

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