Difference between Torque and Couple:

Torque is a special kind of force that can rotate an object about an axis. While a force is described as a pull or push, it is better to think of torques as a twist

In a special case when applied force vectors add to zero, then the force is called a couple and their moment is called torque. Thus, the rotational force produces no moment between the two forces. The moment of a couple is called a torque.

When a driver turns a steering wheel, he exerts two equal but opposite forces on it. The two forces form a couple. The turning effect of a couple is the sum of the moment of the two forces. The moment of a couple is called Torque.

 

(i) Difference between stable and neutral equilibrium

States of equilibrium:

There are three states of equilibrium

• Stable equilibrium
• Unstable equilibrium
• Neutral equilibrium

 

1. Stable equilibrium:

When the Centre of gravity of a body lies below the point of the suspension or support, the body is said to be in stable equilibrium. For example, a book lying on the table is in stable equilibrium.

Explanation:

A book lying on a horizontal surface is an example of stable equilibrium. If the book is lifted from one edge and then allowed to fall, it will come back to its original position. Other examples of stable equilibrium are bodies lying on the floor such as a chair, table etc.

Reason for stability:

When the book is lifted, its centre of gravity is raised. The line of action of weight passes through the base of the book. The torque due to the weight of the book brings it back to the original position.

 

2. Unstable Equilibrium:

When the Centre of gravity of a body lies above the point of suspension or support, the body is said to be in unstable equilibrium.

Example

Pencil standing on its pint or a stick in a vertically standing position.

Explanation:

If a thin rod standing vertically is slightly disturbed from its position it will not come back to its original position. This type of equilibrium is called unstable equilibrium, other examples of unstable equilibrium are vertically standing cylinders and funnels.

Reason for instability:

When the rod is slightly disturbed its Centre of gravity is lowered. The line of action of its weight lies outside the base of the rod. The torque due to the weight of the rod toppled it down.

 

3. Neutral equilibrium:

When the Centre of gravity of a body lies at the point of suspension or support, the body is said to be in neutral equilibrium.

Explanation:

If a ball is pushed slightly to roll, neither will it come back to its original position nor it will roll forward rather it will remain at rest. This type of equilibrium is called a neutral equilibrium.

 

Reason for neutral equilibrium:

If a ball is rolled, its Centre of gravity is neither raised nor lowered. This means that its Centre of gravity is at the same height as before.