Physics paper 2 help needed

[InvisionFree]

I have my physics paper this wednesday.
Im having trouble in finding out the magnitude of the resultant force.

http://www.xtremepapers.com/CIE/Interna ... 4_qp_2.pdf
In this paper, Q1 part (b) the examiner expects us to find the answer by using cosine rule

AND

http://www.xtremepapers.com/CIE/Interna ... 5_qp_2.pdf
In this paper, Q3 part (c) the examiner expects us to find the resultant force by using simple sin, cos and tan.

I know both the methods but I am not sure when to use the cosine and sine rule and when to use simple sin, cos and tan method.
I'm really confused.
Someone please help me out.
Thanks-

 

[CaptainDanger]

You can use any method for the answer... If your answers are matching marking scheme you don't have to worry...

 

[Konfused]

i cant get the answer given by the mark scheme :unknown: for the first question .. can some one plz show me how? thnx

 

[InvisionFree]

You can use any method for the answer... If your answers are matching marking scheme you don't have to worry...

You get different answers by using different methods. Try it yourself.

Someone please help me out.

 

[zenyatales]

This is how I did it.

For the first question,
LET the upper force = 6N
the lower force = 8N (horizontal direction)

Then, resolve the upper force , 6N to get x component = 6cos40 = 4.596 N
And, y component = 3.856 N

Then, total force in the horizontal direction, X = 8 + 4.569 = 12.569 N
and, total force in the vertical direction = 3.865 N

Then , use Result force, R = square root of ( X^2 + Y^2 ).

 

[zenyatales]

This gives R = 13.2 N . Which is the answer in the mark scheme.

 

[Abdulrab]

can anyone please help me in understanding the concept of leading and lagging between two waves !!! i would be thankful if anyone try to clear it with the help of a diagram !! thanks

 

[wahab305]

simply the best way.....firstly replace lag with phase difference
secondly.... u know that 180 degree means that they are in totally opposite direction.............count down the square on the graph for example 3 blocks occupy half wavelength
now simple 180:3 and the angle it lag behind ratio:x
find the value of x
plot the wave in those found blocks...

 

[Abdulrab]

can u make a diagram i didnt even understand a single word !! thnks

 

[Konfused]

This is how I did it.

For the first question,
LET the upper force = 6N
the lower force = 8N (horizontal direction)

Then, resolve the upper force , 6N to get x component = 6cos40 = 4.596 N
And, y component = 3.856 N

Then, total force in the horizontal direction, X = 8 + 4.569 = 12.569 N
and, total force in the vertical direction = 3.865 N

Then , use Result force, R = square root of ( X^2 + Y^2 ).

now i get what i was doing wrong thnx a lot!!

 

[zenyatales]