Physics - Class X - Chapter No.6 - Questions And Answers

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EQUILIBRIUM

IMPORTANT QUESTIONS OF PAST PAPERS

1. Write down three states of equilibrium. (2009)
2. Prove that moments of couple is equal to the force and arm of couple. (2015)
3. Describe two condition of equilibrium. (2015)
4. Define moment arm. (2015)
5. Define torque. (2016)

IMPORTANT SYMBOLS: 

• Force = F
• Moment Arm (distance from one point to another point) = d & r
• Mass = m
• torque = ɽ

IMPORTANT FORMULA’S:

Torque = force × moment arm
τ = F × d

Questions/ Answer

Q.1: Define equilibrium and write down the types of equilibrium.
Ans:EQUILIBRIUM: A body said to be in equilibrium if the forces acting on it must be cancel the effect of each other.

“OR”

When two or more forces act on a body, such that their resultant effect on the body is zero and the body retains its state of rest or of uniform motion then the body is said to be in equilibrium.
Example: A book lying on the table. All forces which are acting are zero.

TYPES OF EQUILIBRIUM: There are two types of equilibrium.
1- Static Equilibrium: A body remains at rest under the influence of different forces it is called static equilibrium.
For example: book lying on the table.

2- Dynamic Equilibrium: If the body is moving with uniform velocity under the influence of different forces it is called dynamic equilibrium.
For example paratrooper falls down with uniform velocity.

Q.2: Define center of gravity.
Ans: CENTER OF GRAVITY: The point inside and outside the body where the whole weight of the body appears to act is called the center of the gravity.

Q.3: Write down the states of equilibrium with suitable example.
Ans: There are three states of equilibrium.
1- Stable Equilibrium: A body is said to be in stable equilibrium if it returns to its original position when it is slightly disturbed. The centre of gravity is raised and then comes to its original position.
Example: A cone is standing on its base.




  2- Unstable Equilibrium: A body is said to be in unstable equilibrium if it does not return to its original position when it is slightly disturbed. The centre of gravity is lowered and then does not come to its original position.
Example: A cone balanced on its top is unstable equilibrium.




3- Neutral state of Equilibrium: In equilibrium the centre of gravity of the body neither raised nor lowered when it is disturbed. Every time the centre of the gravity of body changed due to the new position of the body.
Example: If a ball is rolled on the surface of earth. The centre of the gravity of ball neither raised lowered when the ball is disturbed.

Q.4: Define Moment Arm.

Ans:  MOMENT ARM: It is the perpendicular distance between the axis of rotation and the line of the action of the forces.


Q.5: Define torque and write down its formula and unit.
Ans:TORQUE: The turning effect of a force on a body is called torque.

“OR”

&The product of force and moment arm is called torque.

Formula:

Torque = force × moment arm

τ = F × d

Unit: The unit of torque is Nm

Q. 6. Write down first condition and second condition of equilibrium.
Ans:FIRST CONDITION OF EQUILIBRIUM: The sum of all the forces acting along X- axis is zero the sum of all forces acting along Y-axis is zero.
On X-aix: If the sum of all forces acting on the body along x-axis.

∑F = 0
F_1x+ F_2x = F_3x+ F_4x
∑Fx = 0

On Y-axis: If the sum of all forces acting on the body along y-axis.

∑F = 0
F_1y+ F_2y = F_3y+ F_4y
∑Fy = 0

SECOND CONDITION OF EQUILIBRIUM: The sum of all torque acting on a body is always equal to zero.

Clockwise = anticlockwise
τ₁ + τ₂ = τ₃ + τ₄
∑ τ = 0

Q.7: Define parallel forces, like parallel forces and unlike parallel forces.
Ans:PARALLEL FORCES: When the number of forces acts on a body and if there are parallel they are called parallel forces.
LIKE PARALLEL FORCES: If two parallel forces have same direction is called like parallel force.
UNLIKE PARALLEL FORCES: If two forces have opposite direction is called unlike parallel forces.

Q.8: Prove that moments of couple is equal to the force and arm of couple.
Ans:COUPLE: A pair of equal, parallel and unlike forces having different lines of actions is called couple.
Calculation of Moment of Couple: Consider two equal unlike parallel forces, each of magnitude F, acting at A and B. The torques of moment of two forces is given by;

The moment of the force at A= F × OA

The moment of the force at B= F× OB

Both these moments have 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 × OA + F× OB
Moment of the Couple = F (OA × OB)
Moment of the Couple = F × AB

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

Prepared by: Sir Waseem