Statics: Chapter 6: X Physics,home tuition Karachi

KarachiQ1. What do you mean by Resultant Force?

Ans: Resultant Force:

The sum of all forces acting upon a body is called Resultant Force. If two forces acting

On a body then sum of both is called Resultant forces. If we take the difference of two

Forces, then their resultant is also called Resultant Force.

Explanation:

(i) Consider the diagram in which, two opposites but equal forces :” F1” and “F2” are

acting upon a block. The block remain at rest under the influence of these force,

because the magnitude of forces equal.

F₁ F₂

(ii) Consider another body, two forces F1 and F2 are acting upon it in opposite

direction. Here the magnitude of F2 is greater, so the body will move towards

greater force.

F₁ F₂

(iii) Consider another case, TWO forces “F1 and “F2” acting on the body making a

certain angle with each other then. The resultant F formed between F1 and F2.

F₁

F

F₂

(iv) Now consider the case in which two forces F1 and F2 acting on a body with

certain angle. In this case, the forces can be represented in magnitude and

direction by two adjacent sides of a parallelogram. The resultant force is a vector

represented by the diagonal “FR”. This is called Parallelogram of forces.

F₁ F₁

F₂

F₂ F₂

(a) (b)

Q2. Define equilibrium. Write the types of equilibrium.

Ans. EQUILIBRIUM:

When the effect of all the forces acting on a body in such a way that it does not

Change its state of rest or motion, the body is said to be in equilibrium OR “In other

words, we can say that bodies in equilibrium may remain in state of rest or move with

uniform velocity”.

TYPES OF EQUILIBRIUM:

STATIC EQUILIBRIUM:

i. If a body remains at the state of rest then it said to be Static Equilibrium For

example

ii. A book laying on a table.

Iii .Stationary building and bridges etc.

DYNAMIC EQUILIBRIUM:

If a body moving with uniform velocity then the body is said to be in dynamic

equilibrium For example;

a. A car moving with uniform velocity.

b. A train moving with uniform velocity etc.

Q3. State the first condition of Equilibrium.

Ans. FIRST CONDITION OF EQUILIBRIUM:

If two or more than two forces acting upon a body in such away that their line of

Action is straight line and they cancel the effect of each other then the body will remain

In equilibrium.

“In other words, a body will b in equilibrium if the resultant of all forces acting on it, is

zero. “

EXPLANATION:

The force acting on a position x-axis of a body equal to the force acting on negative

x-axis of body or the sum of all force acting on x-axis of a body is zero. Then body

is said to be in equilibrium.

F_{1 x} + F_{2 x} + F_{3 x} ……………….. = 0

S F x = 0

Where S denotes the sum and Fx force acting on x-axis.

if the sum of the all force acting on y-axis of a body is zero then the body is said to be

in equilibrium.

F_{1 y} + F_{2 y} + F_{3 y} ………………… = 0

S F_y = 0

Where F_y is force acting on y-axis.

Q4. State the second condition of Equilibrium.

Ans: SECOND CONDITION OF EQUILIBRIUM:

If two or more than two force are acting on a body in such a way that some force try

to move the body in clockwise direction and some forces try to move the body in anti

clockwise direction. The sum of all these forces becomes zero then body will remain in

equilibrium.

OR

A body will be in equilibrium when the algebraic sum of all the torques

Acting upon it is zero.

St = 0

Where t is denoted torque F

Q5. How many types of equilibrium. Explain each.

Ans. STATE OF EQUILIBRIUM:

There are three state of equilibrium

i. Stable Equilibrium

ii. Unstable Equilibrium

iii. Neutral Equilibrium F

STABLE EQUILIBRIUM:

In stable equilibrium if the body slightly disturbed the center of gravity of the

body is raised an then comes to its origin it position.

Example:

A book is lying horizontally on the surface

of the table. If this book is slightly raised

from the one corner, the center of gravity

of book slightly raised then by leaving the c

book the center of gravity of the book comes

to its original position.

UNSTALBE EQUILIBRIUM:

In unstable equilibrium , if the body is slightly disturbed the center of gravity is

lowered and does come its original position.

Example:

If a pencil is kept in vertical and it is slightly

lowered and the pencil falls down and center

of gravity does not come to its original position

NEUTRAL EQUILIBRIUM:

In neutral equilibrium the center of gravity of the body neither raised nor

lowered when it is disturbed. Every time center of gravity of body is changed

due to the new position of the body.

Example:

If a ball is rolled on the surface of earth, the

center of gravity of ball acquires the new c c

position every time according to the position

of the ball. The CG of ball neither raised

nor lowered when the ball is disturbed. Neutral equilibrium

Q6. Define Centre of Gravity.

Ans. CENTRE OF GRAVITY:

The center of gravity of a body is that point of the body at which whole weight of a

body always acts and through which resultant force due to earth’s attraction passes.

Q7. Give the center of gravity of following bodies.

(i) Sphere (ii) Triangular Plate (iii) Cylinder (iv) Plane square rectangular

Ans: Sphere:

Centre of gravity of sphere is center of sphere. CG

TRIANGULAR SPHERE:

CG. of the triangular sphere plate is intersection of medians. CG

CYLINDER:

CG. of cylinder is mid-point of axis.

CG

CG

PLATE SQUARE RECTANGULAR:

CG. of plate square rectangular is intersection of diagonals.

Q8. Write the experiment to find the center of gravity of irregular shaped object.

Ans: CENTRE OF GRAVITY OF IRREGULAR SHAPED OBJECT:

i. Suppose, we have to find the center of

gravity of an irregular shaped metallic plate.

ii. Drill few small holes near the edges of the plate.

iii. using a hole “A”, suspend the plate from a nail

fixed horizontally in a wall.

iv. The plate will come to rest in a position so that

its center of gravity is vertically below the point

of suspension.

Q9. What do you mean by moment of force arm?

Ans: MOMENT OF FORCE ARM:

The perpendicular distance between the axis of rotation of the body and line of the act

in of the force is called Force Arm.

Q10. Define Torque, Positive Torque and Negative Torque.

Ans. TORQUE:

The turning effect of a force about the axis of rotation is called Torque.

OR

“If a body is freely rotating about an axis by the application of the force turning effect

is produce in the body the turning is called Torque.”

OR

“The produce of force and Moment arm of Force arm is called Torque.”

Mathematically:

Torque = Force x Moment of Force arm

t = ½F½½r½ Sin 0

Where t is torque, |F| is magnitude of force, Jr| moment of force arm.

NEGATIVE TORQUE:

If a force is applied on a body then it rotates in clockwise direction then it is called

Negative Torque.

POSITIVE TORQUE:

If a force is applied on a body then it rotates in anticlockwise direction. The torque is

called Positive Torque.

Q11. On what factors does torque is depends?

Ans. To understand, the factors on which the torque depends.

If “F” force is applied on a point “B” of door then it not

easily open. Similarly, force “F” applied on a point “A”

door then door opens easily So it is proved that if the

perpendicular direction between force and axis of rotation

is greater, torque is greater. So torque, depends upon following two factors;

The distance from the axis of rotation to the line of action of force or moment

Arm of the force.

The magnitude of the force.

Q12. What is meant by maximum toque and minimum torques?

Ans: MAXIMUM TORQUE:

When the angle between force and force arm is 90^o, then torque is maximum.

MINIMUM TORQUE:

When the angle between force and force arm is “0”, the torque is minimum.

Q13. Give the units of Torque in different systems.

Ans.

SYSTEM	UNITS
(I) C.G.S. System (ii) M.K.S. System (iii) S.I.System (iv) F.P.S. or B.E. System	Dynes-cm Newton-meter Newton-meter Lbs-ft.