Q.A cyclist riding in a straight line at a constant speed of 20 km/h on a level road is in:

Equilibrium describes a state where all forces acting on an object are balanced, resulting in no change in its state of motion. There are two primary types of equilibrium:

• Static Equilibrium: An object is at rest and remains at rest. The net force acting on it is zero.
• Dynamic Equilibrium: An object is in motion at a constant velocity (constant speed in a straight line) and continues to move at that constant velocity. The net force acting on it is also zero.

In the given scenario, the cyclist is riding in a straight line at a constant speed of 20 km/h. This means their velocity is constant, as both the speed and direction are unchanging. When an object moves at a constant velocity, its acceleration is zero. According to Newton's first law of motion, if the acceleration is zero, the net force acting on the object must also be zero. This condition precisely defines dynamic equilibrium.

Let's examine the options:

• (a) static equilibrium: This is incorrect because the cyclist is moving, not at rest. Static equilibrium applies to objects that are stationary.
• (b) dynamic equilibrium: This is correct. The cyclist is moving at a constant velocity, meaning the net force acting on them is zero, which is the definition of dynamic equilibrium. …

To understand the state of the cyclist, we first need to grasp the concept of equilibrium in physics, particularly as it applies to motion. Equilibrium describes a state where an object experiences no net change in its motion. This occurs when the sum of all external forces acting on the object is zero.

There are two primary types of equilibrium:

1. Static Equilibrium: This is the state where an object is completely at rest and remains at rest. In this condition, its velocity is zero, and the net force acting on it is also zero. A book lying motionless on a table is an example of static equilibrium.

2. Dynamic Equilibrium: This is the state where an object is moving, but its velocity is constant. "Constant velocity" means both its speed and its direction are unchanging. If the velocity is constant, then the acceleration is zero. According to Newton's First Law of Motion, if the acceleration is zero, the net force acting on the object must also be zero. A satellite orbiting Earth at a constant speed in a perfect circular path (if we ignore air resistance and other minor forces) or a car cruising at a steady speed on a straight highway are examples of dynamic equilibrium.

Now, let's apply this to the cyclist in the question. The cyclist is described as "riding in a straight line at a constant speed of 20 km/h on a level road."

• "Constant speed" means the magnitude of the velocity is not changing.
• "In a straight line" means the direction of the velocity is not changing.
• Together, "constant speed in a straight line" implies that the cyclist's velocity is constant.

Since the cyclist's velocity is constant, their acceleration is zero. Consequently, the net force acting on the cyclist (including the bicycle) is zero. Because the cyclist is in motion (not at rest) and the net force is zero, they are in a state of dynamic equilibrium.

Let's examine why the other options are incorrect:

• (a) static equilibrium: This is incorrect because static equilibrium applies to objects that are at rest. The cyclist is moving at 20 km/h, which is not at rest. …