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