- #1

**1.**R^2 = F1^2 * F2^2 + 2*F1*F2*COS(the angle between F1 and F2)

**2.**The second is about the parallelogram rule, it says that the two vectors are added and their summation is the magnitude of the resultant.

Which one is correct?

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter Bassel AbdulSabour
- Start date

- #1

Which one is correct?

- #2

tnich

Homework Helper

- 1,048

- 336

If I correctly understand you, both are correct. You seem to have the sign wrong in the equation. ##R^2=F_1^2+F_2^2-2F_1F_2\cos(\theta)##

1.R^2 = F1^2 * F2^2 + 2*F1*F2*COS(the angle between F1 and F2)

2.The second is about the parallelogram rule, it says that the two vectors are added and their summation is the magnitude of the resultant.

Which one is correct?

Last edited:

- #3

tnich

Homework Helper

- 1,048

- 336

Here is a diagram showing my understanding of the problem.If I correctly understand you, both are correct. You seen to have the sign wrong in the equation. ##R^2=F_1^2+F_2^2-2F_1F_2\cos(\theta)##

- #4

- 16,934

- 6,739

That would typically not be ”the angle between the forces”. The angle between two forces in the same direction would typically be zero, whereas your convention would be pi.Here is a diagram showing my understanding of the problem.

View attachment 230677

- #5

tnich

Homework Helper

- 1,048

- 336

I agree. I am trying to interpret what the OP has written.That would typically not be ”the angle between the forces”. The angle between two forces in the same direction would typically be zero, whereas your convention would be pi.

- #6

- 5,824

- 1,129

Note that the angle-between-the-vectors (tails together, as in the parallelogram method of addition) is not the interior angle in the triangle (in the tail-to-tip method of addition).

The parallelogram rule for adding vectors is true, independent of the metric.

That tells you how to add two vectors... with the tails together, construct a parallelogram, and draw from the common tail to the opposite corner.

That's the resultant vector.

Getting the magnitude of the vector-sum is a different step [see rule 1].

- #7

sophiecentaur

Science Advisor

Gold Member

2020 Award

- 25,682

- 5,110

That's the old "Cos Rule" which we all did at school. Using the Supplementary Angle (as with vectors, you just get a change of sign.Here is a diagram showing my understanding of the problem.

View attachment 230677

Cos(x) = -Cos(π-x)

Share: