void Start()
    {
        checkpoints = GetComponentsInChildren<Checkpoint>();
        sw.Start(); //stopwatch starts
        Load();
        checkpointsReached = 1;
        nextCheckpoint = checkpoints[checkpointsReached];
    }

internal void CheckpointReached(Checkpoint checkpoint)
    {
        if (nextCheckpoint != checkpoint)
            return;

        checkpointsReached++;

        if (checkpoint.isFinish)
        {
            sw.Stop();  //stopwatch stops after reaching Finish
            EndRace();
            return;
        }

        if (checkpointsReached == checkpoints.Length)
        {
            checkpointsReached = 0;
        }

        nextCheckpoint = checkpoints[checkpointsReached];
    }

void FixedUpdate()
    {
        Rigidbody2D rb = GetComponent<Rigidbody2D>();
        rb.AddForce(transform.up * speedForce);  //acceleration


        //turning and stuff
        rb.angularVelocity = Input.GetAxis("Horizontal") * torqueForce;
        rb.velocity = ForwardVelocity() + RightVelocity() * driftFactor;