1
WB JEE 2017
MCQ (Single Correct Answer)
+2
-0.5
Change Language
A particle with charge Q coulomb, tied at the end of an inextensible string of length R metre, revolves in a vertical plane. At the centre of the circular trajectory, there is a fixed charge of magnitude Q coulomb. The mass of the moving charge M is such that $$Mg = {{{Q^2}} \over {4\pi {\varepsilon _0}{R^2}}}$$. If at the highest position of the particle, the tension of the string just vanishes, the horizontal velocity at the lowest point has to be
A
0
B
$$2\sqrt {gR} $$
C
$$\sqrt {2gR} $$
D
$$\sqrt {5gR} $$
2
WB JEE 2017
MCQ (Single Correct Answer)
+2
-0.5
Change Language
A bullet of mass 4.2 $$ \times $$ 10$$-$$2 kg, moving at a speed of 300 ms$$-$$1, gets stuck into a block with a mass 9 times that of the bullet. If the block is free to move without any kind of friction, the heat generated in the process will be
A
45 cal
B
405 cal
C
450 cal
D
1701 cal
3
WB JEE 2017
MCQ (Single Correct Answer)
+2
-0.5
Change Language
A particle with charge e and mass m, moving along the X-axis with a uniform speed u, enters a region where a uniform electric field E is acting along the Y-axis. The particle starts to move in a parabola. Its focal length (neglecting any effect of gravity) is
A
$${{{2m{u^2}} \over {eE}}}$$
B
$${{{eE} \over {2m{u^2}}}}$$
C
$${{{mu} \over {2eE}}}$$
D
$${{{m{u^2}} \over {2eE}}}$$
4
WB JEE 2017
MCQ (Single Correct Answer)
+2
-0.5
Change Language
A unit negative charge with mass M resides at the mid-point of the straight line of length 2a adjoining two fixed charges of magnitude +Q each. If it is given a very small displacement x(x << a) in a direction perpendicular to the straight line, it will
A
come back to its original position and stay there
B
execute oscillations with frequency $${1 \over {2\pi }}\sqrt {{Q \over {2\pi {\varepsilon _0}M{a^3}}}} $$
C
fly to infinity
D
execute oscillations with frequency $${1 \over {2\pi }}\sqrt {{Q \over {2\pi {\varepsilon _0}M{a^2}}}} $$
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