How to get the instantaneous fps rate when using Javafx AnimationTimer

I am making a game which runs according to physical laws: namely F = ma. Objects in the game, change their velocities and positions according to the force experienced by them.
I am using the AnimationTimer class. The game is 2-dimensional.
I have a class Ball:

public class Ball extends javafx.scene.shape.Circle {
    int m; //mass
    int vX, vY; // velocity componenets

    void feelForce (int forceX, int forceY) {
        vX += forceX * t/m;
        vY += forceY * t/m;
        setCenterX (getCenterX() + vX * t);
        setCenterY (getCenterY() + vY * t);
    }
}

The problem I face is that I don't know how to define t. The number of frames per second might vary throughout runtime, and I want to know how to find that value, each time feelforce() is called.

public void start(Stage stage) throws Exception {
    //have overridden the constructor in class Ball. Had not shown it to keep it short
    Ball player = new Ball(400.0, 300.0, 30.0, Color.TOMATO); 
    Pane main = new Pane(player);
    Scene scene = new Scene(main);
    stage.setScene(scene);
    stage.show();

    AnimationTimer gamePlay = new AnimationTimer() {
        public void handle(long now){
            player.feelForce(Math.random() * 10, Math.random() * 10);
            // I use MouseEvents to get force values, but have used random numbers here, for brevity
        }
    };
    gamePlay.start();
}

If this is no possible/efficient using AnimationTimer, a solution using another class would also be appreciated.
P.S: The reason I am using AnimationTimer instead of Timeline, is that a fixed rate is not necessary.