PlayerMovement.cs

using UnityEngine;

public class PlayerMovement : MonoBehaviour
{

	// This is a reference to the Rigidbody component called "rb"
	public Rigidbody rb;

	public float forwardForce = 2000f;  // Variable that determines the forward force
	public float sidewaysForce = 500f;  // Variable that determines the sideways force

    private void Start()
    {
		rb.AddForce(0, 700, 500);
		rb.useGravity = true;
	}

    // We marked this as "Fixed"Update because we
    // are using it to mess with physics.
    void FixedUpdate()
	{
		// Add a forward force
		rb.AddForce(0, 0, forwardForce * Time.deltaTime);

		if (Input.GetKey("d"))  // If the player is pressing the "d" key
		{
			// Add a force to the right
			rb.AddForce(sidewaysForce * Time.deltaTime, 0, 0, ForceMode.VelocityChange);
		}

		if (Input.GetKey("a"))  // If the player is pressing the "a" key
		{
			// Add a force to the left
			rb.AddForce(-sidewaysForce * Time.deltaTime, 0, 0, ForceMode.VelocityChange);
		}

		if (rb.position.y < -10f)
        {
			FindObjectOfType<GameManager>().EndGame();
        }
	}

}