 # Turning Effect of Forces

## 9th Class Physics Chapter 4 Exercise Short Questions with Answers

Q 4.2 : define the following :

(i)

Resultant Vector :

” A resultant vector is a single vector that has the same effect as the combined effect of all the vectors to be added .”

(ii)

Torque :

” The turning effect of a force is called torque or moment of the force .”

Symbol : t

Formula : t = r F

Unit : Newton metre (Nm)

(iii)

Centre of Mass :

” Centre of mass of a system is such a point where an applied force causes the system to move with rotation .”

The Centre of mass of a system moves as if its entire mass is confined at that point .

Example :

(iv)

Centre of Gravity :

” A point where the whole weight of the body appears to act vertically downward is called Centre of gravity of a body .”

Q 4.3 : Differentiate the following :

(i)

Like and Unlike parallel forces

(ii)

Torque and couple

(iii)

Stable Equilibrium and Unstable Equilibrium

Q 4.4 : How head to tail rule helps to find the resultant of forces ?

” It is a graphical method used to find the resultant of two or more forces is called Head to Tail Rule .”

→ In this method , vectors are added graphically by following steps / method :

• The tail of 2nd vector coincide with the head of 1st vector and so on .
• The resultant of all vectors is obtained by coinciding the tail of 1st vector with the head of last vector .

Q 4.5 : How can a force be resolved into its rectangular components ?

Resolution of a Force :

” A force can be resolved inti its perpendicular components by drawing its two mutually perpendicular components .”

• Consider a force F is making an angle  θ with x-axis .
• This force can be resolved into its rectangular components by drawing its two mutually perpendicular components .
• The magnitude of these rectangular components is :

Fx = F cos  θ

Fy = F sin  θ

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Q 4.6 : When a body is said to be in equilibrium ?

A body is in equilibrium if it satisfies both conditions of equilibrium .

• 1st Condition : A body is in equilibrium if net force acting on it is zero .

ΣF = 0

ΣFy = 0

ΣFx = 0

• 2nd Condition : A body is said to be in equilibrium if the resultant torque acting on it is zero .

Σt = 0

Q 4.7 : Explain the first condition for equilibrium .

1st Condition for Equilibrium :

” A body is said to satisfy first condition for equilibrium if the resultant of all the forces acting on it is zero .”

Mathematically :

F1 + F2 + F3 + . . . . . .. . . + Fn = 0

ΣF = 0

Q 4.8 :Why is there a need of second condition for equilibrium if a body satisfies first condition for equilibrium ?

• In above example , two equal and opposite forces are acting on a body but a long different lines .

This body is in equilibrium according to 1st Condition .

• But body can rotate , Therefore , we need another condition of equilibrium to satisfy that body is in complete equilibrium condition .

Q 4.9 : What is second condition for equilibrium ?

2nd Condition for Equilibrium :

” A body satisfies second condition for equilibrium when the resultant torque acting on it is zero .”

Mathematically :

Σt = 0

Q 4.10 : Give an example of moving body which is in equilibrium .

• A Paratrooper coming down with terminal velocity (constant velocity ) is in equilibrium .
• All the forces acting on it are equal to zero , So it satisfies 1st Condition for Equilibrium .

Q 4.11 : Think of a body which is at rest but not in equilibrium .

If we throw a ball upward , at its highest position , It is at Rest for a moment .

• But force of gravity is also acting on it to produce acceleration .

Thus , the ball is at rest but not in equilibrium .

Q 4.12 : Why cannot a body be in equilibrium due to single force acting on it ?

Reason :

Because a single force is not balanced . It produces acceleration in the body .

Thus, under the influence of single force , a body cannot be in equilibrium .

Q 4.13 : Why the height of vehicles is kept as low as possible ?

Height of vehicle is kept low and their bottom is made heavy .

Reason :

• To lower their centre of gravity .
• To increase their stability .

Q 4.14 : Explain what is meant by stable , unstable and neutral equilibrium . Give one example in each case .

Stable Equilibrium :

” A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position .”

Example : A book laying on a table .

Unstable Equilibrium :

” If a body does not return to its previous position when sets free after a slight tilt , it is said to be in un-stable equilibrium .”

Example : A pencil standing on its point .

Neutral Equilibrium :

” If a body remains in its new position when disturbed from its previous position , it is said to be in a state of neutral equilibrium .”

Example : Rolling Ball .

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