Physics2D polyfill to support ForceMode (Unity 3D). Source: http://answers.unity3d.com/questions/574864/rigidbody2d-and-forcemodeaddvelocity.html

Physics2DExtensions.cs

using UnityEngine;

public static class Physics2DExtensions {
	public static void AddForce(this Rigidbody2D rigidbody2D, Vector2 force, ForceMode mode = ForceMode.Force) {
		switch (mode) {
		case ForceMode.Force:
			rigidbody2D.AddForce(force);
			break;
		case ForceMode.Impulse:
			rigidbody2D.AddForce(force / Time.fixedDeltaTime);
			break;
		case ForceMode.Acceleration:
			rigidbody2D.AddForce(force * rigidbody2D.mass);
			break;
		case ForceMode.VelocityChange:
			rigidbody2D.AddForce(force * rigidbody2D.mass / Time.fixedDeltaTime);
			break;
		}
	}
	
	public static void AddForce(this Rigidbody2D rigidbody2D, float x, float y, ForceMode mode = ForceMode.Force) {
		rigidbody2D.AddForce(new Vector2(x, y), mode);
	}
}