MCQs Chapter4 9th Physics New Book
MCQs Chapter4 9th Physics New Book will be solved in detail
4.1 A particle is simultaneously acted upon by two forces of 4 and 3 newtons. The net force particle is:
(a) 1 N (b) between 1 N and 7 N (c) 5N (d) 7 N
Option (b) is correct.
Explanation:-
If the the forces are in same direction their sum will be 7 N and id they are in opposite direction their sum will ne 1N.
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4.2 A force F is making an angle of 60 degree with x-axis. Its y-component is equal to:
(a) F (b) F Sin60 (c) F Cos60 (d) F Tan60
Option (b) is correct.
Explanation:-
X-component will be F Cos 60 and Y-component will be F Sin 60.
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4.3 Moment of force is called:
(a) moment arm (b) couple (c) couple arm (d) torque
Option (d) is correct.
Explanation:-
Another name of torque is moment.
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4..4 $$If\;F_1\;and\;F_2\;are\;the\;forces\\acting\;on\;a\;body\;\;and\;\tau\;is\\the\;torque\;produced\;in\;it,$$ the body will be completely in equilibrium, when:
$$(a)\;\sum F\;=\;0\;and\;\sum\tau\;=\;0\\(b)\;\sum F\;=\;0\;and\;\sum\tau\;\neq\;0\\(c)\;\sum F\;\neq0\;and\;\sum\tau\;=\;0\\(d)\;\sum F\;\;\neq0\;and\;\sum\tau\;\neq\;0$$
Option (a) is correct
Explanation:-
For complete equilibrium both linear and angular acceleration must be zero. Sum of all the forces is zero means linear acceleration is zero and sum of all the torques is zero means angular acceleration is zero
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4.5 A shopkeeper sells his articles by a balance having unequal arms of the pans. If he puts the weight in the pan having shorter arm, then the customer:
(a) loses (b) gains
(c) neither loses nor gain (d) not certain
Option (a) is correct.
Explanation:-
Principle of level and torque is applied here. The product of weight of goods and longer arm must be equal to the product of standard weight and shorter arm. The costumer gets less than the actual weight because when the shopkeeper places the weight on the shorter arm, less goods are needed on the longer arm to balance.
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4.6 A man walking on a tight rope. He balances himself by holding a bamboo stick horizontally. It is an application of:
(a) law of conservation of momentum
(b) Newton’s second law of motion
(c) principle of moments
(d) Newton’s third law of motion
Option (c) is correct.
Explanation:-
Clock wise torque must be equal to anti clock wise torque. Bamboo stick increases his moment arm. Hence small forces produce larger torque which the man can easily control.
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4.7 In stable equilibrium, the centre of gravity of the body lies:
(a) at the highest position (b) at the lowest position
(c) at any position (d) outside the body
Option (b) is correct.
Explanation:-
Centre of gravity is at lowest position because small tilt rises the centre of gravity and gravity pulls it back down to balance again.
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4.8 The centre of mass of a body:
(a) lies always inside the body
(b) lies always outside the body
(c) lies always on the surface of the body
(d) may lie within, outside or on the surface
Option (d) is correct.
Explanation:-
The centre of mass of a body may lie within or outside or on the surface of the body because it depends on its mass and shape of the body. For regular shapes centre of mass lies within the body and for curved shapes it lies out the body. Centre of mass of a hollow metal bowl cut in half lies on its surface.
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4.9 A cylinder resting on its circular base is in:
(a) stable equilibrium (b) unstable equilibrium
(c) neutral equilibrium (d) none of these
Option (a) is correct.
Explanation:-
A cylinder resting on its circular base in in stable equilibrium because when it is slightly tilted, it comes back to its original position.
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4.10 Centripetal force is given by:
$$(a)\;r\;F\\(b)\;r\;F\;Cos\;Ɵ\\(c)\;\frac{m\;v^2}r\\(d)\;\frac{m\;v}{r^2}$$
Option (c) is correct.
Explanation:-
$$Centripetal\;acceleration\\is\;equal\;to\;\;\frac{v^2}r\\We\;know\;that\\F=\;m\;a\\Hence\\F\;=\;m\;\frac{v^2}r\\F=\;\frac{m\;v^2}r$$
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