Physics Class 9th Notes

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

Ans. (a) in the 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)

(a)    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 the table on the block in an upward direction these forces have some line of action and balance each other.

(b)   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.

(c)    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 the addition of forces in detail:

Ans:-force is a vector quantity and forces can be added with each other by vector algebra. The vector we get after the addition is known as the resultant vector. Some special cases of the addition of vectors are given below.

1)      When the angle b/w forces are 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 a magnitude of resultant forces as shown.

2)       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

3)   When angled/w forces are other than 00  1800. When angle b/w forces are 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.

4)      Head to tail rules:– that method in which we combine the tail of the 2nd vector with the head of 1stvector and finally we combine the tail off is a vector is joined with the head of the last vector whose direction is toward the last vector and this is our resultant vector this method of the edition of the vector is known as a head-tail rule.

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