# Class 9th Physics Notes Chapter 4 TURNING EFFECT OF FORCES

Class 9th Physics Notes Chapter 4 TURNING EFFECT OF FORCES 9th class notes for 9th class Physics

## Class 9th Physics Notes

### Q.1 explains the various forces acting on a body?

Ans. (a) in earth gravitation field, the attracts everything to ward its center the gravitational force acting on each particle of body is parallel as shown in fig (a)
(b) In figure (b) two a block is shown on a table. Two forces are acting on it. Its weight “w” in downward direction while the normal force of table on block in upward direction these force have some line of action and balance each other.
(c) In figure “c” two forces “p” and “Q” are shown which are acting at points “A” and “B” these forces are known as parallel forces.
(d) In figure (d) two force “l” and “m” are shown which are acting at points “c” and “D” these forces are known as “ anti-parallel” forces.

### Q.2 Explain addition of forces in detail:

Ans:-force is a vector quantity and forces can be add with each other by vector algebra. The vector we get after addition is known as resultant vector. Some special cases of addition of vectors are given below.
When angle b/w forces is zero (Q=0).when the angle b/w any two forces is zero then these vectors will be parallel to each other in this case we add the magnitude of individual forces to get magnitude of resultant forces as shown.
When forces are anti parallel I Q = 1800:- when two forces are anti parallel then in this case we subtract the magnitude of individual forces from each other to get the magnitude of resultant forces. As shown above in fig 5 + -10 = -5
When angle/w forces is other than 00ε 1800. When angle b/w forc es is other than 00 and 1800 then we add these vector by different methods in which “head to tail” is mostly used. Head to tail rule method is given below.
Head to tail rules:- that method in which we combine the tail of 2nd vector with the head of 1st vector and finally we combine the tail of is vector is joined with the head of last vector whose direction is toward last vector and this is our resultant vector this method of edition of vector is known as head tail rule. ### Q.3 Explain resolution of vectors in detail?

Ans. Resolution of vectors:-the process of splitting of a force into two or more components is known as resolution of vectors.
Explanation:- usually a vector is resolved into two such components which are perpendicular to each other and these components are known as rectangular components .the components and the components parallel to y- axis is known as y –component. ### Q.4 what is torque? On what factors does at depends?

Ans. torque:- the toning effect produced in a body due to applied force is known as torque or moment of force(OR) the measure of an object tendency to rotate about some point is called torque or moment of force.
Explanation:- consider a door as shown when we apply a force an door, then it begins to rotate because of torque. The axis about which the door beignets to rotate is known as axis of rotation. The point at which force is applied is known as point of action and the perpendicular distance b/w point of action of force and axis of rotation known as momentum arm.
Factors upon which torque depends:- we now that mathematically torque is given by the product of moment arm and applied force rotates anti – clock wise then it is known as anti-clock wise torque and it is taken as positive.

### Q.5 explains principal of moment in detail?

Ans. principle of moment:- it stated that a body will be in state of equilibrium, if the sum of all clock wise moments balance the sum of anti-clock wise moments taken over the same pivot.
Explanation:- A meter rod is shown in figure which is balance on a wedge such that w1 and w2 are suspended from right side and w3 is suspended from left side of meter rode let آ1, آ2 and آ3 Are the torques produced by w1 , w2 and w3respectively.
Now we see that آ1” and آ2 acts as clock wise torque and آ3 acts as anti-clock wise torques. Since meter rods in is state of equilibrium. ie.
Clock wise torque = anti clock wise torques Q.6 Define and explain center of gravity and center of mass?

Ans.(1) Center of gravity:- the point where whole weight of the body appears to act is known as center of gravity.

Explanation:- we known that everybody consists of large number of practical  And each particle has a definite weight. Now the sum of weights of all particles is equal to whole weight to the body appears to act is known as center of gravity of that body.

2center of mass:- we know that everybody is made of large number of small practical’s which has a definite mass and the masses of all particle makes mass of the body if we apply a force on body and body starts motion. Then the point at which we have applied the force is called center of mass for example center of mass of a meter rod lies at midpoint.        .c.m

Q.7 where the C.G of regular and irregular bodies is located?

Ans. (1) C.G of regular bodies:-center of gravity of regular bodies is located at their geometrical center.

• Centre of gravity of spherical bodies is located. at its center as shown in gig (a)
• Centre of gravity of uniform meter rod is also lies at Centre of meter rod. Fig (e)
• The center of gravity of square and rectangle intersect each other. Fig “b” € “c”
• The Centre of gravity of triangular shaped bodies is located as shown in fig “d”.

(2) CG of irregular bodies:- to find center of gravity of an irregular shaped body we make some holes at the edges of the body then we suspend the body by every hole me by me and draw vertical lines with the help of passing a thread as shown the body in figure. The lines draw are also shown in figure after join these lines we get lines AA’,BB’ and CC’ which intersect each other at point “O” and point of intersection is the C.G of this irregular body. Class 9th Physics Notes

Q.8 what is equilibrium? Also give its conditions?

Ans. Equilibrium:- the state of rest or uniform motion o a body is called equilibrium (or) if a body possess no acceleration then the body is called in equilibrium.

There are two conditions for a body to be in state of equilibrium. They are given below.

1st condition of equilibrium:- according to 1st condition of “A” body will be in state of equilibrium if the sum of forces acting on a body is zero i.e (£F = o)

For example a bulb is hanging from the ceiling with the help of a thread then the weight “w” of bulb is acting in downward direction while the tension “T”
in thread is acting in upward direction in this case both forces balance each other i.e resultant of both force is equal to zero a result the bulb remains in state of equilibrium.

2nd condition of equilibrium­:-  according to this condition “a body will be in state of equilibrium if the sum of anti-clock wise torques is equal to the sum clock wise torques “OR” a body will be in equilibrium it the sum of all torques is equal to zero.    £T = 0

A meter rod is shown in figure at which two force “p” I ´are acting at points “A” and “B” though the 1st condition is satisfied but it still rotates and body is not in equilibrium. Not get state of equilibrium we soul make clock wise torque equal to anti –clock wise torque then we will get equilibrium.

When there is no change in translation motion of a body then the equilibrium is known translational equilibrium. Of static objects is called static translational equilibrium eg book .the equilibrium is bodies moving with constant velocity is known as dynamic translational equilibrium. If 2nd condition is satisfied on bodies in rotation then equilibrium is known as rotational equilibrium.

Q9 Define and explain couple?

Ans. Couple:- A couple consists of two parallel forces that are equal in magnitude but opposite in direction eg turning of water top turning of door key and turning of car wheel steering wheel.

Explanation:- suppose a steering wheel of a car is gripped by two hands form two point “A” and “B” as give in figure. When both hands apply forces F1 and F2 of equal magnitude but in opposite direction. Then the wheel begins to rotate. Such forces applied by both hands, whose magnitude is equal but opposite the direction is called  couple the shortest distance b/w two couple forces is called “arm of couple.

Q.10 explains state of equilibrium in detail?
Ans. State of equilibrium (stability) _. In different states of equilibrium we discuss the stability of an object. That how a body in equilibrium behaves and how it changes the position of center of gravity when it is disturbed by some external force.

• (1) Stable state of equilibrium:- if a body comes back to sits initial position after being disturbed then it is called stable state of equilibrium for example when we left a book form one side laying on the table and then leave it the book will come back to its original position. Because when the book is lifted form one side its center of gravity raised from point “C” to “c” and comes back to its original state due torque produced by its weight.
• Unstable state of equilibrium:- when a body is slightly disturbed and it does not come back to its original state then it is called unstable state of equilibrium. For example when a pencil in vertical position is disturbed, then it falls down and does not come back to its original position because be in this case the C.G becomes lower from point “C” and the line of action of weight is directed outside of pencil and it falls down.
• Neutral state of equilibrium:- when a body is slightly disturbed and its center of gravity neither lowers nor raised from its original position then such a body is called din state of neutral equilibrium. For example we roll a ball on ground. Then its center of gravity remains at its original position but body rolls forward.

## CONCEPTUAL QUESTIONS

Class 9th Physics Notes

Q.3 why is at more difficult to lean back word?

Ans. it is more difficult to lean back word because in such cases the center of gravity of the body is disturbed due which the stability decreases and it becomes more difficult to remain in state of equilibrium.

Q.4 as we know that:  = sinQ

Ans. which show  that torque depends upon moment arm “r” Grater the “r” greater will be torque produced that’s why  handles are not put near hinges they are put away from hinges to rotate them easily.

Q.5 why does a helicopter has a second rotor on its tail?

According to 2nd condition of equilibrium if the sum of all forces acting on a body is equal to zero then the body will be in state of equilibrium. That’s why there is a 2nd rotor on the tail of helicopter, the torque produced by 1st rotor is cancelled by the torque produced by 2nd rotor and this helicopter remains in state of equilibrium.

Q.6 why is it better to use a long spanner rather than a short one to tighten a nut on a bullet?

Ans. as we know that torque depends upon the moment arm “r” an “f” as long spanner has greater moment of arm. There for it is better to use long spanner rather than a short spanner to tighten a nut on a Bolt easily.

Q. 8 A girl is seated safety and steady canoe but when she tries to stand up the canes capsize explain this in term of c. m and stabile?

Ans. As the girl stand up the center of gravity and the e.m of body gets higher than its original position as result the stability decreases and this canoe capsizes.

Q10 the gravitational force acting on a satellite is always direction toward center of earth, does this force exit torque on satellite?