How to Make a RigidBody not accelerate?

I'm trying to make a moving platform that slides up and down. I'm new to Unity so this might not be the best way to do this. I need to make the platform not eccelerate so it doesn't fly off the screen. Does anyone know a better way to do this or a way to stop acceleration?

Here is my Code:

Code (CSharp):

1. using UnityEngine;
2.  
3. public class MovingPlatforms : MonoBehaviour
4. {
5.     private Vector3 lowestPos;
6.  
7.  
8.  
9.     private void Start()
10.     {
11.         lowestPos = transform.position;
12.     }
13.  
14.  
15.     private void Update()
16.     {
17.      
18.         if (transform.position.y >= lowestPos.y + 3)
19.         {
20.             GetComponent<Rigidbody2D>().gravityScale = 1;
21.         }
22.         if (transform.position.y <= lowestPos.y)
23.         {
24.             GetComponent<Rigidbody2D>().gravityScale = -1;
25.         }
26.  
27.     }
28.  
29.  
30.  
31. }

 

I realized that this plan wouldn't work and that when another Rigidbody lands on top, It falls down due to the gravity increase. So instead of using gravity does anyone have a better way of doing this?