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As usual start by reading Chap 7 and do the examples without looking at solutions (paraphrase the concept). 

The level of complexity in the problems really increase in this chapter and you may find that some of the examples are hard to do without help.

Chapter 7

In the next few pages, I will elaborate more on the material.

Interacting objects. (7.1-7.2)

Let us look at how different objects interact. In the following image, a runner uses a starting block to propel herself forward. Let us examine the forces on the runner first and then we can look at the starting block.  

Neglecting the air, the runner is only in contact with the starting block (right foot in figure). Since this is a surface, it will lead to two forces, a normal and a friction force. The friction force has to be static since there is no relative motion between feet and the block. The direction of the static friction is uncertain at first. When solving such problems, you should make an educated guess for the direction of the friction. If later after solving you find a negative number, you will know that your guess was wrong.

The static friction wants to oppose motion and my guess is that the runner must be pushing the block upward to get a friction downward. So the free-body diagram for the runner is

This is an example of propulsion. The runner is pushing on the block and the block pushes back. With friction and normal force both pointing to the right, we can get the biggest force to the right (which is the goal of the runner).

What would the free body diagram on the starting block look like? The starting block touches the runner and the ground. Each will provide a normal and a friction force so there will be multiple similar forces. This can get really confusing!

Do not hesitate to label forces! Here we will use the full notation F_{A\;on\;B}

The objects in the problem are:

Label Object
R Runner
B Block
G Ground
EE Entire Earth

With this notation the free body diagram for the runner becomes

and the free body diagram for the starting block will be

All the forces on the free-body diagram of the runner are "on R" while all the forces on the free-body diagram of the starting block are "on B". Make sure you put the forces at the correct place!
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