1. How do we know that the force of the string on the bob does not become zero when it is moving?

Ryan makes four force diagrams because there are four instances in which forces are being exerted. Both when the cart is held and let go and when the bob is moving and at rest. The differences between Ryan's diagrams and mine were that his arrow lengths were different then mine, because when i did this on my own, i thought the force of the string on the bob while it was moving would be at zero, now however, i know that this is not true because if there is a force being exerted on an object, it will always exert a force on that object. Meaning that because in a previous diagram the string and the bob had a force diagram, then the would still have it while the bob was moving.

4. Explain how you know the force of the string on the bob has decreased but is not zero. Your answer is not convincing. A good explanation will involve the cart.
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You know the the force of the string on the bob has decreased, but is not 0 because the cart is being pulled by the string connected to the bob and we know that because of Newton's second law objects will always exert the same amount of force on one another so long as they are touching.