**Q.3: Define the axis of rotation? Give any one example?**

**Ans: Axis OF ROTATION:**

Some bodies cannot move from place but they can rotate about a fixed line or axis, called its axis of rotation. A force acting on such a body can only produce rotation.

**Example:**

When we apply a force on door it can rotate as it cannot move as a whole along a straight line. Then an axis about which a body is free to rotate, is called axis of rotation.

**Q.4: Define moment arm?**

Ans: The moment arm can be defined as, “It is the perpendicular distance between the axis of rotation and the line of the action of the forces.”

**Q.5: Define torque or moment? Write down its formula and units?**

**Ans: TORQUE OR Moment:**

The turning effect of a force on a body is called torque or moment of force and is equal to the product of force and moment arm.

**Formula:**

From the following formula we can find the torque or moment.

Torque = Force X moment arm

tT=F Xr

where “r” = Tau (r) means torque

“F” = Applied force

“r“ = Moment arm, it is vector quantity

**Unit Of Torque Or Moment:**

The S.I. Unit of torque is Nm.

T = F x r

T = Nm

**Q.6: Write down the factors on which torque depends?**

**Ans: TORQUE DEPENDS ON THE FOLLOWING FACTORS:**

- The torque is directly proportional to the applied force.
- The torque is directly proportional to the moment arm.

**Q.7: Define Center of gravity?**

Ans; The point inside or outside the body where the whole weight of the body appears to act is called the center of gravity of the body.

**Q.8: Write down the position of center of gravity of the following
object?**

**Ans: PosiTion OF CENTER OF GRAVITY:**

- Uniform Rod
- Plate (square, rectangle or parallelogram in shape)
- Rectangular block or cube
- Circular plate
- Triangular plate
- Cylinder

**Q.9: How can we find the center of gravity of irregularly shaped body?**

Ans: We can find the center of gravity of a irregular shaped body by using plumb line. Plumb line is a device with which we can find the center of gravity of irregular shaped very easily.

There are two forces which is acting on plumb line.

- Weight of plumb line
- Tension in the string

**Q.10: How can you find the center of gravity of an irregular metal sheet
or card sheet?**

Ans: By avery simple method we can find the center of gravity of irregular shaped bodies. This method is as under

Drill three small holes near the edge of the plate whose center of gravity is to be located. Suspend the plate from a nail fixed horizontally in a wall using one of the holes say “A”. When the plate is at rest suspend a plumb line from the nail. Draw a line “Aa” on the plate along the plumb line. The center of gravity lines somewhere on the line “Aa”.Repeat the procedure with hole “8” on the nail. Again the center of gravity must lie somewhere on the line “Bb”. The only point common on the line “Aa” and “Bb” therefore this point must be the center of gravity. If the plate is suspended using third hole C, the line also pass through point “G”.

**Q.11: What is a couple? Calculate the moment of the couple?**

**Ans: COUPLE OF FORCES:**

A pair of equal parallel and unlike forces having different lines of action is called a couple.

**Calculation Of Moment Of Couple:**

Consider two equal unlike parallel forces each of magnitude “F” acting at “A” and “B” the torques or moments of two forces are given by. The moment of the force “F” at A = F x OA. The moment of the force “F” at B = F x OB. Both these moments have the same direction i.e. counter clockwise so the total moment of the two forces is equal to the sum of the two moments.

**Q.12: What is a coupie? Calculate the moment of the couple?**

Ans: COUPLE OF Forces:

A pair of equal parallel and unlike forces having different lines of action is called a couple. Calculation Of Moment Of Couple: Consider two equal unlike parallel forces each of Ff magnitude “F” acting at “A” and “B” the torques or moments of two forces are given by:

The moment of the force “F” at A =F x CA

The moment of the force “F” at B = F x OB

Both these moments have the same direction i.e. counter clockwise so the total moment of the two forces is equal to the sum of the two moments.

- moment of the
couple = F x OA+F x OB
- F (OA + OB)

- F X AB

Moment of the couple is equal to the product of one of the forces and the perpendicular distance between the line of action of two forces. The perpendicular distance between the two forces is called the arm of the couple.

Thus the torque or moment of a couple is equal to the product of either force and arm of the couple. Whenever a couple acts on a body there is rotation only. It should be noted that a couple cannot be balanced by single force. It can be balanced by an equal and opposite couple.