Select the correct option :
a)Work done by static friction is always 0
b)Work done by kinetic friction can be positive also
c)Kinetic energy of a system cannot be increased without applying any external force on the system
d)Work energy theorem is valid in non inertial frames also
Please give examples/explanation for the a) part.

To tackle the question regarding the work done by static friction, let's delve into the concept of friction and work in physics. The correct option is a) Work done by static friction is always 0. This statement is true, and I’ll explain why that is the case.

Understanding Static Friction

Static friction is the force that prevents an object from starting to move when a force is applied. It acts in the opposite direction to the applied force, ensuring that the object remains at rest. The key point here is that static friction does not do any work when the object is not moving.

Work and Its Definition

In physics, work is defined as the product of the force applied to an object and the distance over which that force is applied, in the direction of the force. Mathematically, it is expressed as:

• Work (W) = Force (F) × Distance (d) × cos(θ)

Here, θ is the angle between the force and the direction of motion. If the object does not move, the distance (d) is zero, which leads to:

• W = F × 0 = 0

Static Friction in Action

Consider a scenario where you push a heavy box on a floor, but it doesn’t move because the static friction force is equal to the force you are applying. In this case, while you are exerting a force, the box remains stationary, and thus:

• The distance moved by the box is zero.
• Consequently, the work done by static friction is zero.

Real-World Example

Imagine trying to push a car that is parked on a flat surface. If the car does not budge, the static friction between the tires and the ground is counteracting your push. Since the car hasn’t moved, the work done by the static friction force remains zero, even though the force is present.

Why This Matters

This concept is crucial in understanding how forces interact in various scenarios. It highlights that not all forces result in work being done, particularly when there is no movement involved. This principle is foundational in mechanics and helps in analyzing systems in equilibrium.

In summary, the statement that "work done by static friction is always 0" holds true because static friction acts to prevent motion, and without movement, no work is performed. This understanding is essential for further studies in physics, especially when dealing with forces and energy in different contexts.