Hi,
You are right. I assume we use Time.deltaTime here because we call the Rigidbody method in the Update method. Aaaactually, it is supposed to be called in FixedUpdate. However, since this course caters to beginners, Rick didn’t want to introduce too many concepts at once. If you are already familiar with Unity, feel free to rework the code in your project. 
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- Forum User Guides : How to mark a topic as solved