Would you explain your thought process? I looked at it again and still think it's T - mg. I'm using g = +9.81 m/s/s, which is the convention the IB uses. If you're thinking of g = -9.81 m/s/s, then you're correct that it would be T + mg. It's more common to see people treating g as positive, but maybe there are some who make it negative. This is different from what most people do, but there's nothing innately wrong with that approach. We certainly *could* do physics this way, and as long as we're consistent, we'd get the same answers. It's really just a matter of convention. Here's my thought process--let me know if you think this is on the right track, or if you still disagree. When the ball is at the bottom, the tension force pulls up, and the gravity force pulls down. We know the net force is toward the center, because the ball is moving in a circle. So there must be more upward force than downward force. For example, if the ball feels 15 N upward and 10 N downward, then the net would be 15 N - 10 N = 5 N (directed upward). That's very obvious, but there's a reason I showed this simple example: the point I wanted to make is that T and mg represent the *magnitudes*, such that T and mg are both positive values. In the example, it would be T = 15 N and mg = 10 N. So T - mg = 15 - 10 = Fnet. It's equivalent to writing Fnet = T + (-mg), because mg points downward. But we need to write -mg to capture the downward direction because mg alone is a positive value.
Beautifully explained.
thanks a lot for video
thanks so much for this!!!
The example you give in minute 11 is wrong. The net force when the ball is down is the mg + T
Would you explain your thought process? I looked at it again and still think it's T - mg. I'm using g = +9.81 m/s/s, which is the convention the IB uses. If you're thinking of g = -9.81 m/s/s, then you're correct that it would be T + mg. It's more common to see people treating g as positive, but maybe there are some who make it negative. This is different from what most people do, but there's nothing innately wrong with that approach. We certainly *could* do physics this way, and as long as we're consistent, we'd get the same answers. It's really just a matter of convention.
Here's my thought process--let me know if you think this is on the right track, or if you still disagree. When the ball is at the bottom, the tension force pulls up, and the gravity force pulls down. We know the net force is toward the center, because the ball is moving in a circle. So there must be more upward force than downward force. For example, if the ball feels 15 N upward and 10 N downward, then the net would be 15 N - 10 N = 5 N (directed upward). That's very obvious, but there's a reason I showed this simple example: the point I wanted to make is that T and mg represent the *magnitudes*, such that T and mg are both positive values. In the example, it would be T = 15 N and mg = 10 N. So T - mg = 15 - 10 = Fnet. It's equivalent to writing Fnet = T + (-mg), because mg points downward. But we need to write -mg to capture the downward direction because mg alone is a positive value.