Physics Unit 6

Today we started learning about how to find the acceleration of objects by using Newton's Second  Law of Physics, which states that the net force of an object equals its mass times its acceleration (Fnet=ma). This whole new system is both terrifying and confusing for me, but I hope that this post acts as a good review for me, and hopefully by looking over my notes again, I gain a stronger understanding.

So, there are three steps to take: 1) draw free body diagrams, 2) find acceleration of the system, and 3) choose one mass to find T. Using these three steps, I will show an example problem to help clarify what you are supposed to do.

Problem: There is a box of 50 kg on top of a frictionless table being pulled by a piece of rope which is in a pulley (changes direction of force). At the end of the rope is another box hanging over the side of the table with a mass of 10kg. Find the acceleration of the system and its tension of the string.

1. Draw free body diagrams.

2. Find acceleration of the system.
Ok, so for a while I didn't understand how to find the Fnet, and I'm not entirely sure if what I think is correct actually is correct, but oh well. The way I think you're supposed to find the Fnet is entirely based on your free body diagram. With it, you can see what forces you can use and which you can't. The normal force and the weight of the 50kg box are equal, which means that they have a difference of zero newtons. The 10kg box, on the other hand, has a much higher weight than it does tension, since the box is accelerating down. This means that when pluggin in the answers for the Fnet, you would subtract the smaller value from the bigger one, in this case that would mean that the tension is being subtracted from the weight of the 10kg box. Since the 50kg box is connected to the same string as the 10kg box, they have the same amount of tension, which would be added to the Fnet, since the variables are on the same axis. As for the rest of the equation, you can just plug everything in based on the information you already know.

3. Choose one mass to find the tension.
Once you find the acceleration, you use the same Fnet=ma equation to find the tension. You only had to choose one of the two boxes to find the tension, but I wanted to show that the tensions were the same, so I solved for both. Anyway, for the 50kg box, like I said earlier, its normal force and weight are equal, so the only Fnet I had for it was its tension. The 10kg box, however, had its weight and tension, so I used both. Once you figure out what exactly to plug in, the actual solving of the equation is relatively simple.



I'm not entirely sure if I was even right about my explanations on how to solve everything, but on the bright side, at least I now have a somewhat stronger understanding of how to solve problems like these.