Roll-A-Ball tutorial error when attaching movement script to ball

I am new to Unity (and a beginner C# programmer), and am going through the Roll-A-Ball tutorial. I am at the point where I am attaching a script that handles the movement input for the ball. After completing the code and trying to playtest, I get this error:

"Assets/_Scenes/Scripts/PlayerController.cs(14,27): error CS0120: An object reference is required to access non-static member `UnityEngine.Rigidbody.AddForce(UnityEngine.Vector3, UnityEngine.ForceMode)'".

I triple checked and I typed everything correctly, so I am at a loss as to what to do. Any help would be appreciated, thank you.