Home Courses Contact About




16.(JEE Main 2020 (Online) 2nd September Morning Slot )

If speed V, area A and force F are chosen as fundamental units, then the dimension of Young’s modulus will be

(A) FA–1V0

(B) FA2V–1

(C) FA2V–2

(D) FA2V–3

Correct answer is (A)

Y = k [F]x [A]y [V]z

[M1L1T –2] = [MLT–2]x [L2]y [LT–1]z

[M1L1T –2] = [M]x [L]x+2y+z[T]–2x–z

Comparing power of M, L and T

x = 1 ……(1)

x + 2y + z = –1 ……(2)

–2x – z = –2 ……(3)

After solving

x = 1

y = –1

z = 0

Y = FA–1V0

17.(JEE Main 2020 (Online) 8th January Morning Slot )

The dimension of stopping potential V0 in photoelectric effect in units of Planck's constant 'h', speed of light 'c' and Gravitational constant 'G' and ampere A is :

(A) h1/3 G2/3 c1/3 A–1

(B) h0 c5 G-1 A-1

(C) h2/3 c5/3 G1/3 A–1

(D) h2 G3/2 c1/3 A–1

Correct answer is (B)

V0 hPcQGRIS

[V0] = [M1L2T–3A–1]

[c] = [L1T–1]

[h] = [M1L2T–1]

[G] = [M–1L3T–2]

[I] = [A]

[M1L2T–3A–1] = [MP–R L2P+Q+3R T–P–Q–2R AS]

Comparing dimension of M, L, T, A, we get

P – R = 1 ; 2P + Q + 3R = 2

– P – Q – 2R = – 3 ; S = – 1

P = 0, Q = 5, R = –1, S = –1

V0 h0 c5 G-1 A-1

18.(JEE Main 2019 (Online) 8th April Evening Slot )

If surface tension (S), Moment of inertia (I) and Planck's constant (h), were to be taken as the fundamental units, the dimensional formula for linear momentum would be :-

(A) S1/2I1/2h0

(B) S3/2I1/2h0

(C) S1/2I1/2h-1

(D) S1/2I3/2h-1

Correct answer is (A)

We know,

surface tension (S) = F L = [ M L T 2 ] [ L ]

[S] = [ M T 2 ]

Moment of inertia (I) = mr2

[I] = [ M L 2 ]

Planck's constant (h) = E f = Et

[h] = [ M L 2 T 1 ]

Also linear momentum (p) = mv = [ M L T 1 ]

Now we have to express p in terms of s, I and h.

Let, [P] = [Sa Ib hc]

[ M L T 1 ] = [ M T 2 ] a [ M L 2 ] b [ M L 2 T 1 ] c

[ M L T 1 ] = [ Ma + b + c L2b +2c T- 2a - c ]

By comparing the dimensions of both sides, we get

a + b + c = 1 .........(1)

2b +2c = 1 ..............(2)

- 2a - c = -1 ...................(3)

By solving those three equations we get,

a = 1 2

b = 1 2

c = 0

linear momentum [p] = [S1/2I1/2h0]

19.(JEE Main 2019 (Online) 11th January Evening Slot )

If speed (V), acceleration (A) and force (F) are considered as fundamental units, the dimension of Young,s modulus will be:

(A) V 2A2F2

(B) V 4A 2F

(C) V 4A2F

(D) V 2A2F 2

Correct answer is (C)

We know,

Young's modulus (Y) = F A Δ l l

[Y] = [ M L T 2 ] [ L 2 ] = [ ML-1T-2]

Let [Y] = [V]x [A]y [F]z

[ ML-1T-2] =

[LT-1]x [LT-2]y [MLT-2]z

[ ML-1T-2] =

[ Mz Lx + y + z T-x -2y - 2z

For dimensional balance, the dimension on both sides should be same.

So, z = 1

x + y + z = -1

x + y = -2 ........(1)

and -x -2y - 2z = -2

x + 2y = 0 ...........(2)

By solving those two equations we get,

x = -4 and y = 2

[Y] = V 4A2F1

20.(JEE Main 2019 (Online) 9th January Evening Slot )

Expression for time in terms of G(universal gravitional constant), h (Planck constant) and c (speed of light) is proportional to :

(A) h c 5 G

(B) c 3 G h

(C) G h c 5

(D) G h c 3

Correct answer is (C)

Let t Gx hy cz

   [t] = [G]x [h]y [c]z    . . . . . (1)

We know,

F = G M 2 R 2

  G = F R 2 M 2

  [G] = [ M L T 2 ] [ L 2 ] [ M 2 ]

[G] = [ M 1 L 3 T 2 ]

Also,

E = hf

  [h] = [ E ] [ F ]

= [ M L 2 T 2 ] [ T 1 ]

= [ M L 2 T 1 ]

[C] = [ M o L T 1 ]

From equation (1) we get,

[ M o L o T 1 ] = [ M 1 L 3 T 2 ] x [ M L 2 T 1 ] y [ M o L T 1 ] z

[ M o L o T 1 ] = [ M x + y L 3 x + 2 y z T 2 x y z ]

By comparing the power of M, L, T

x + y = 0

  x = y

3x + 2y + z = 0

  5x + z = 0 . . . . . (2)

2x y z = 1

   3x z = 1  . . . . (3)

By solving (2) and (3), we get,

x = 1 2 = y and z = 5 2

  t G h C 5