School bags are typically designed with broad straps rather than thin ones. This design feature is intended to:

Correct Option

The correct option is

Reduce the pressure exerted on the shoulders by increasing the area of contact.

Explanation

The physical concept of Pressure, which is defined as the force exerted per unit area. Mathematically, it is expressed as:

Pressure (P) = Force (F) / Area (A)

This relationship indicates that for a constant force, pressure is inversely proportional to the area of contact ($P \propto 1/A$).

Detailed Analysis

• The Force Involved: The force acting on the student's shoulders is the weight of the school bag, which remains constant regardless of the strap design.
• Role of Surface Area: According to the formula $P = F/A$, increasing the surface area ($A$) over which the force is distributed results in a decrease in the pressure ($P$) exerted on that surface.
• Application to Straps: Broad straps provide a larger surface area of contact with the shoulder compared to thin straps. Consequently, the weight of the bag is distributed over a wider area, significantly reducing the pressure exerted on the shoulder muscles and skin. This minimizes discomfort and the risk of injury.
• Contrast with Thin Straps: Thin straps have a smaller contact area, which would concentrate the same weight into a smaller region, resulting in high pressure and potential pain.

Analysis of Incorrect Options:

• Increase the pressure exerted on the shoulders to keep the bag in place.: Increasing pressure would cause more pain and discomfort, which is contrary to the design purpose.
• Decrease the contact area to minimize material usage.: While decreasing contact area might minimize material, it would increase pressure, making the bag difficult to carry.
• Increase the friction between the strap and the shoulder to prevent slipping.: Friction prevents the strap from slipping, but the primary reason for broadening the strap is pressure distribution, not friction enhancement.

Key Takeaway

Pressure is inversely proportional to the area of contact. Increasing the contact area for a given force reduces the pressure, making it easier to support heavy loads like school bags.