Moving and Jumping

using UnityEngine;
using System.Collections;

public class Movement : MonoBehaviour {

    public float speed = 5f;
    public float jumpForce = 8f;

    public Transform groundCheck;
    float groundedRadius = .2f;
    public LayerMask groundMask;

    Rigidbody2D _rigidbody2D;

    bool grounded;

    void Start ()
    {
        _rigidbody2D = rigidbody2D;
    }

    void FixedUpdate()
    {
        grounded = Physics2D.OverlapCircle (groundCheck.position, groundedRadius, groundMask);
    }
    void Update ()
    {
        float x = Input.GetAxis ("Horizontal");
        _rigidbody2D.velocity = new Vector2 (speed * x, rigidbody2D.velocity.y);

        if(Input.GetButtonDown("Jump") && grounded)
        {
            _rigidbody2D.AddForce (new Vector2(0, jumpForce), ForceMode2D.Impulse);
        }
    }
}