Unit 6: Forces That Accelerate

Today we learned a lot more in depth on how to calculate forces using FBD's (free-body-diagrams). It got a lot more complicated when we added pulleys, friction and Fnet. We also learned three rules or steps to follow when dealing with these types of problems:
1. Draw FBD's
2. Find acceleration (a) of the system
3. Choose one mass to find the tension

Here is an example where I use some of these steps to solve the problem:

In this picture, there are two objects, both on either end of a string and in between the objects, is a pulley. A good thing to remember is that a pulley changes the direction of your force, so even though the force of the pulley is going down, it is pulling the blue object to the right. The first step is to draw an FBD of the image, so I have done that in the picture below.

As you can see, I have done an FBD for both objects, each in corresponding colors. The blue object has both the force of gravity and normal force, which balance each other out on the y-axis, but has tension going to the right, which is unbalanced which means that the blue object is accelerating to the right. In this problem, we are not taking into account friction. On the red object, there are unbalanced forces. The tension is less than the force of gravity which means the red object is accelerating downwards. Always remember, if there is no surface, then there is no normal force. Notice how the T's are the same length. That is because the objects are connected by the same string, and therefore the string has the same tension. The next step if to find the acceleration of the system.

In this step we use math (ewww). So far, the only formula we have is Fnet = ma. Luckily, that is the only one we need. Fnet is the difference between all of the forces. After that, it is all math and substitution.