M1 help please, urgent

A particle of mass 0.8 kg is moving under the action of two forces P and Q. The force P has magnitude 4 N and the force Q has magnitude 6 N. The angle between P and Q is 110°, as shown. The resultant of P and Q is F. Find

a the angle between the direction of F and the direction of P.

b the magnitude of the acceleration of the particle.

I have tried to work the mag of F by using the cosine rule and then use the sine rule to work out the angle i got 8.27 N as the mag for F and the angle as 43
cheers , why doesnt it work as i dont know..?

Reply 1

TenOfThem

18

Original post by noodlemon

A particle of mass 0.8 kg is moving under the action of two forces P and Q. The force P has magnitude 4 N and the force Q has magnitude 6 N. The angle between P and Q is 110°, as shown. The resultant of P and Q is F. Find

a the angle between the direction of F and the direction of P.

b the magnitude of the acceleration of the particle.

I have tried to work the mag of F by using the cosine rule and then use the sine rule to work out the angle i got 8.27 N as the mag for F and the angle as 43
cheers , why doesnt it work as i dont know..?

Well, it is difficult for us to know since the question includes the line "as shown" and you have not shown us

However, I guessed what the diagram looks like and I have the magnitude of F as 5.97N

Reply 2

imcurious

2

As far as I'm concerned is

a) 70.94 degrees

b) 5.965N

What I used was just cosine and sine rule. That's it.

Reply 3

noodlemon

OP

5

sorry, im panicking as the exams 2moro.. yes youre right, but how did you get the answer, i used the cosine rule to find mag of F, and then sine ruled it, but the book says the angle is 70.9, i got43...

Reply 4

TenOfThem

18

Original post by noodlemon

sorry, im panicking as the exams 2moro.. yes youre right, but how did you get the answer, i used the cosine rule to find mag of F, and then sine ruled it, but the book says the angle is 70.9, i got43...

You are using the wrong triangle

To get the resultant you have to have one force start where the other one ends

Pick Q up and move it across to the right hand end of P

Now you have a triangle with 70 rather than 110 and the cosine rule works just fine

Reply 5

noodlemon

OP

5

Original post by TenOfThem

You are using the wrong triangle

To get the resultant you have to have one force start where the other one ends

Pick Q up and move it across to the right hand end of P

Now you have a triangle with 70 rather than 110 and the cosine rule works just fine

OOH i see!! ah thank you so much! i just couldnt get my head around that , i suck at force triangles. :P ah it now works i have 5.965 as the resultant force, and thn sine rule it so the angle is 70.9 degrees :smile:

thank you again :biggrin:

Reply 6

TenOfThem

18

Original post by noodlemon

OOH i see!! ah thank you so much! i just couldnt get my head around that , i suck at force triangles. :P ah it now works i have 5.965 as the resultant force, and thn sine rule it so the angle is 70.9 degrees :smile:

thank you again :biggrin:

No problem :biggrin:

M1 help please, urgent

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