Physics with Adults
Newton’s Laws of Motion
This system schema is showing how our objects are interacting with our systems. We have 3 different forces involved with our system schema which means there will be 3 different forces in our free body diagram. 
Newton’s First Law: Every object moves in a straight line unless another force is applied
We are going to focus on Newton's laws and forces with Septa.
What exactly is a System Schema? For those who may not know what a system schema is, it is an outline or diagram showing how objects interact with the system.
What is a free body diagram? A free body diagram are diagrams used to show all the forces acting upon an object.
The system schema above, shows our scenario and how the systems are interacting with the objects/people. As seen above, Gravity is a force for all 4 objects. Human, handle, Earth, and the actual septa bus. A human standing on the bus while the bus is in place, starting from a complete stop, turning a right corner, turning a left corner, and coming back to a complete stop.
This free body diagram and system schema above are both showings, of the exact same scenario. This is what it'll looks like before the bus is moving. A human standing on the bus, no motion is happening, the bus has not yet moved. There are only two forces in our system schema and free body diagram. Force Gravity which is pushing the person up, and force Normal is allowing the person to NOT go through the bus.
Here our system schema and free body diagram looks a bit different. Now the bus has made a left turn. When the bus is turning left, and you hold onto the handle, your body tends to move to the right(the opposite direction in which the bus is turning), because of force Tension. Again, we have force Gravity pushing the person up, and the force Normal allowing the person to NOT go through the bus.
Here we have the bus making a RIGHT turn. When making a right turn, just like the left turn free body diagram, you would go opposite of the way the bus is turning. By pulling on the handle force Tension is moving left. Again like in all free body diagrams, force Gravity is pushing up, and force Normal is pulling up.
The bus is coming back to a complete stop.
The bus makes a complete stop, so there is no more tension and or any other forces being applied. Gravity is holding the person up, and for Normal is allowing the person not to go through the bus.
So.. Think? Without gravity being on every free body diagram, the person would just float up, and without force normal also being on every free body diagram the person would just go through the bus.
Keep this in mind.....
You're a human on a moving septa bus, and you decide to either walk to the front or to the back of a septa bus.
The free body diagram to the right shows if the person were to walk to the front of the bus, when walking to the front of the bus, force Friction will force you to the back of the bus with a pull.
The free body diagram to the right shows, force Friction forcing you to the right. This shows when walking to the back of the bus, which means you are being forced to the front of the bus with a pull.
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