Turn on gravity in 2D from script

Hello everyone,
I want to turn on gravity for platform, if player step on it, because that platform is trap. But I don´t know, how to do it?
Here is a code:

private float time = 2f;    

    void OnTriggerEnter2D(Collider2D col)
    {
        if(col.CompareTag("Player"))
        {

        }
        else if (col.CompareTag("Floor"))
            Destroy(this);
    }

I need it quickly. Thanks