Why do school bags and back packs have broad shoulder straps? Why is it advised to carry school bags on both shoulders instead of one shoulder?

Explanation

### Step-by-Step Solution:

1. **Understanding the Design of Shoulder Straps**:
- School bags and backpacks are designed with broad shoulder straps. The reason for this design is to distribute the weight of the bag more evenly across the shoulders.

2. **Surface Area and Pressure**:
- The broad shoulder straps provide a larger surface area. When a force (in this case, the weight of the bag) is applied over a larger area, the pressure exerted on that area decreases.
- Pressure is defined as force per unit area (Pressure = Force/Area). Therefore, with a larger area, the same weight results in lower pressure on the shoulders.
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