Rigidbody's Gravity not working with Velocity

i have a game object that has a Rigidbody, and i put that object just above another object that serves as their platform. it will move automatically with this

rb.velocity = Vector3.forward * speed;

but the moment I hit the play button, something’s not right. the object’s Y position was 3 for example, but it hovers down like paper. seems like there’s low gravity even if I have check-marked Use Gravity. why does this happen?

Add more mass