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What Does a Spring Scale Measure, Part 2

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Ok, you've done your force analysis. You can check it by comparing it to what we have here.

For this situation, the spring scale accelerates downward. The same two forces exist as before, the tension force and the weight, but now T will be less than the weight of the object. Because the acceleration is downward, we will choose the direction of positive to be down. And so the net force equation is Fnet = mg - T. The tension force will be a negative force, because positive is down, and mg will be a positive force. So we get Fnet = mg-T. Using Newton's Second Law, we get ma = mg-T. Rearranging the equation gets us T = mg-ma. So in this case, the T is smaller than the weight of the object.

Ok we have one more problem to do with the spring scale. I have a little bit different situation here. Here's the scale, I have the scale turned away from you because I don't want you to see the reading yet. And instead of me holding it, I have these strings holding it. One side, we have a string going over a set of pulleys to our 10N bottle of sand. This is the same bottle we were using in our previous demonstrations. On the other side, instead of me holding, we have a second 10N bottle of sand. So we have two equal forces pulling on either direction of the scale. So your problem is to predict what the scale reading is. Is it 10N, is it zero? Do the forces subtract from each other? Do the forces add to each other? So you make your prediction on the electronic form, write down an explanation for the answer you receive, and then click on the link to come back to the video and see what the answer is.

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