Calculate the angle between the displacement vector and the force vector

[Absolutism]

Homework Statement

A force F = 8i-3i N acts on a particle that undergoes a displacement Δr = 2i + j m.
(b) What is the angle between F and Δr? (state your answer to three significant figures)

Homework Equations

Angle = cos-1 (A.B/ AB)

The Attempt at a Solution

W=13.1 J
AB= 8.5*2.23= 19
cos-1 (13.1/19)= 46.4

 

[BitterX]

I will assume that the force is F=8\hat{i}-3\hat{j}

the dot product between F and delta-r:
(8,-3)\cdot (2,1) = 8\cdot 2 + (-3)\cdot 1 = 13
the norm of F:
||F||=\sqrt{8^2 +3^2}
norm of delta-r :
||\Delta r|| =\sqrt{1^2+2^2}
I got 47.12 degrees but it seems you have used different conditions :/

 

[Absolutism]

I got 47.121 another time when I did not round, but even that gave me a wrong answer L: I am not sure what's wrong

 

[BitterX]

If you are sending it to some kind of online checking system:
a. Check to see if they want it in radians.
b. Check if all of the starting conditions are correct.
c. Take into consideration that they make mistakes too.