How to add a force with constant velocity in 2D

I have a object with a rigidbody2D and I need to add a force to it, but the force has to keep the same speed at all times. I tried relativeforce, but it keeps the velocity getting bigger and bigger, and this is not what i want, any suggestions? (it is a 2D game)

If you want constant speed, never faster, never slower, you can do:

rigidbody2D.velocity = rigidbody2D.velocity.normalized * speed;

If you want to limit the speed to some upper limit you can do:

rigidbody2D.velocity = Vector2.ClampMangnitude(rigidbody2D.velocity, maxSpeed);

Note that both lines of code should be executed in FixedUpdate().