# Position and Displacement Vectors- The Watermelon Seed Vector

A watermelon seed has the following coordinates: x = -6.6 m, y = 2.2 m, and z = 0 m. Find its position vector as (a) a magnitude and (b) an angle relative to the positive direction of the x axis. If the seed is moved to the xyz coordinates (8.2 m, 0 m, 0 m), what is its displacement as (c) a magnitude and (d) an angle relative to the positive direction of the x axis?

A) Magnitude was found using (-6.6)^2+(2.2)^2=sqrt(48.4)=6.96m or 7.0m

B) The angle was found using inverse tangent of (2.2/6.6)=18.4 degrees. I subtracted 18.4 from 180 to get 161.6.

C) The magnitude was found using the displacement equation (Δx=X2-X1). 8.2m-(-6.6m)=14.8m

Up until part D, all of the answers are correct in WileyPlus. I have tried multiple ways trying to find this angle however, no success. Perhaps I am looking past the basic point. Please help.

For (c), there is a change in y position too.

I know, it's -2.2m. I forgot to mention it.

TSny
Homework Helper
Gold Member
Hi, Pmand92. Can you show us the details of what you did for part (d)? What did you get for an answer?

Well I tried using the magnitude (14.8 squared) minus 2.2 squared to find a component so I could try the inverse tangent to find the angle. tan^-1(2.2/14.8)=8.5 degrees and then I subtract that from 180, which 171.5. Then at this point, I tried to put it into WileyPlus, which was wrong. So I tried inverse sin with just 2.2/14.8, which didn't work. At this point, I have switched out numbers just trying to see if I could get an answer for the WileyPlus to take the answer.

To be honest, I can't remember all of the ways to find the angle.

TSny
Homework Helper
Gold Member
The displacement is a vector pointing from the initial position to the final position. Draw a sketch to see how that vector is oriented relative to the positive x direction.

I am having a hard time visualizing that.

TSny
Homework Helper
Gold Member
Draw an x-y cartesian coordinate system and locate the initial and final locations (positions) of the seed. The displacement will be a vector pointing from the initial location to the final location. You don't need to worry about the z-coordinate since it's 0 for both the intial and final postitions.