public class Example : MonoBehaviour
{
	public RigidBody body;
	public Vector3 velocity;

	private bool _moveForward;

	private void Update()
	{
		_moveForward = Input.GetKey("W");
	}

	private void FixedUpdate()
	{
		if(_moveForward)
		{
			body.velocity = velocity;
		}
		//Else just let the object slow down on its own using the drag.
	}
}