Explaining Fictitious Forces in Accelerating Carts

[Luca 123]

Let's say there is a cart with a pendulum inside it. The cart then starts accelerating, while an observer on the ground remain still. I was told that the observer, standing still, would not observe any fictitious forces on the pendulum, so how does he explain the pendulum swinging backwards?

 

[Dale]

The pendulum doesn't swing backwards for him. It accelerates forwards a little slower than the cart does.

 

[A.T.]

I was told that the observer, standing still, would not observe any fictitious forces on the pendulum, so how does he explain the pendulum swinging backwards?

The term "observer" refers to a frame of reference. In a ground fixed coordinate system the pendulum never moves backwards.