Physics [Gravitation] Questions for Railway ALP Stage-II Exam : 22nd November 2018

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Q1. A satellite in vacuum
वैक्यूम में एक उपग्रह
(a) is kept in orbit by remote control/ रिमोट कंट्रोल द्वारा कक्ष में रखा जाता है
(b) is kept in orbit by retro-rocket/ रेट्रो-रॉकेट द्वारा कक्ष में रखा जाता है
(c) derives energy form gravitational field/ गुरुत्वाकर्षण क्षेत्र से ऊर्जा प्राप्त करता है
(d) does not require any energy for orbiting/ कक्ष के लिए किसी भी ऊर्जा की आवश्यकता नहीं है
Show Answer
Sol. The satellite revolve due to the gravitational force, that acts as a centripetal force. Without the gravitational force satellite could not be able to revolve around the Earth. 

Q2. Two bodies kept at a certain distance feel a gravitational force F to each other. If the distance between them is made double the former distance, the force will be -
दो शरीर को एक निश्चित दूरी पर एक दूसरे के पास गुरुत्वाकर्षण बल F को महसूस करने के लिए रखा जाता है. यदि उन दोनों के बीच की दूरी को पूर्व दूरी से दोगुना किया जाता है, तो बल होगा- 
(a) 2f
(b) 1/2 f
(c) 4f
(d) 1/4 f
Show Answer
Sol. f^'∝1/r^2
As given in the questions
⇒ f^'∝1/4r^2

Q3. If the Earth stops rotating, the apparent value of g on its surface will 
अगर पृथ्वी घूमना बंद कर देती है, तो इसकी सतह पर g का स्पष्ट मूल्य होगा?
(a) increases at some places and remain the same at other places / कुछ स्थानों पर बढ़ता है और अन्य स्थानों पर समान रहता है
(b) increases everywhere/ हर जगह बढ़ता है
(c) decreases everywhere/ हर जगह घटता है
(d) remain the same everywhere/ हर जगह समान रहता है
Show Answer
Sol. The effect of rotation of earth on g, appears every where but not on poles. So, if earth stops rotating the values of ‘g’ will remain same at poles and increase at other places.

Q4. The shape of our Milky way galaxy is 
हमारी आकाशगंगा का आकार क्या है?
(a) circular/ गोलाकार
(b) elliptical/ दीर्घ वृत्ताकार
(c) spiral/ सर्पिल
(d) None of these/इनमें से कोई नहीं
Show Answer
Sol. The shape of our milky way galaxy is spiral of 100000 –120000 light-years in diameter having 200-400 billion stars. Our solar system is contained into the milky-way galaxy.
This name has been derived from its appearance as a dim milky glowing band arching across the night sky, in which the individual stars cannot be distinguished by the naked eye.

Q5. In the Earth, the weight of a body is maximum at the
पृथ्वी में, शरीर का वजन अधिकतम कहाँ होता है?
(a) North pole/ उत्तरी ध्रुव
(b) South pole/ दक्षिणी ध्रुव
(c) equator/ भूमध्य रेखा
(d) surface/ सतह
Show Answer
Sol. According to the Newton’s law of gravitation, the weight ‘w’ of the body is given by
w = mg
Where, g is the acceleration due to gravity which equals GM/R^2
Where M and R are the mass and radius of the Earth respectively.  As the weight of the body is directly proportional to g. Thus, the weight of the body would be maximum at the centre of the earth.

Q6. The height of a geo-stationary satellite from the earth’s surface is approximately
पृथ्वी की सतह से भू-स्थिर उपग्रह की ऊंचाई लगभग कितनी है
(a) 36,000 km
(b) 42,000 km
(c) 30,000 km
(d) None of these/इनमें से कोई नहीं
Show Answer
Sol. Communication satellites and weather satellites are often provided with the geostationary orbits which are the circular orbits 35786 km above the earth’s equator. Due to this the satellite antennas that communicate with them don’t need to move to track them but can be located permanently at the position in the sky where they stay.

Q7. The atmospheric air is held to the earth by
वायुमंडलीय हवा पृथ्वी पर किस के कारण बनी रहती है?
(a) gravity/ गुरुत्वाकर्षण
(b) winds/ हवा
(c) clouds/ बादल
(d) rotationn of the Earth/ पृथ्वी के घूर्णन
Show Answer
Sol. The atmospheric air is held to the earth by the force of gravity which holds down the atmosphere. The atmospheric air molecules cannot escape the gravitational pull of the Earth as the escape velocity on the Earth’s surface is 11.2 km/s. At the moon’s surface the value of escape velocity is 2.38 km/s so there is no any atmosphere around the moon.

Q8. The minimum number of geostationary satellites needed for uninterrupted global coverage is 
निर्बाध वैश्विक कवरेज के लिए भू-स्थिर उपग्रहों की कितनी न्यूनतम संख्या आवश्यक है?
(a) 3
(b) 2
(c) 4
(d) 1
Show Answer
Sol. Minimum three geostationary satellites are needed for the uninterrupted global coverage.

Q9. We always see the same face of the noon, because 
हम हमेशा दोपहर का एक ही चेहरा देखते हैं, क्योंकि
(a) it is smaller than the Earth/ यह पृथ्वी से छोटा है
(b) it revolves on its axis in a direction opposite to that of the Earth/ यह पृथ्वी के विपरीत दिशा में अपनी धुरी पर घूमता है
(c) it takes equal time for revolution around the Earth and rotation on its own axis / पृथ्वी के चारों ओर क्रांति और अपने अक्ष पर घूर्णन के लिए बराबर समय लगता है
(d) it rotates at the same speed as the Earth around the Sun/ यह सूर्य के चारों ओर पृथ्वी के समान गति से घूमता है
Show Answer
Sol. The synchronous rotation causes the moon to be present just one side to the earth all the time.
This synchronous rotation is the result of tidal locking which occurs when the gravitational gradient makes one side of an astronomical body always face another.
A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This causes one hemisphere constantly to face the partner’s body.

Q10. The term equinox means
इक्वीनॉक्स शब्द का अर्थ क्या है?
(a) the path which the Earth takes around the sun/ वह मार्ग जो पृथ्वी सूर्य के चारों ओर लेता है
(b) the axis of the Earth around which it rotates/ पृथ्वी की अक्ष जिसके चारों ओर घूमती है
(c) when the day and night are of equal duration/ जब दिन और रात बराबर अवधि के होते हैं
(d) the time when the Sun seems to be going round and round in the sky in the arctic but does not go below the horizon/ वह समय जब सूर्य आर्कटिक में आकाश के गोल-गोल घूमता है लेकिन क्षितिज से नीचे नहीं जाता है.
Show Answer
Sol. Around the equinox, the night and day have approximately equal length.
An equinox happens twice a year (around 20th March and 22nd September), when the tilt of the earth’s axis is inclined neither away from nor toward the sun while the centre of the sun remains is the same plane as the Earth’s equator.
At the time of equinoxes the sub-solar point lies on the equator.
This point crosses the equator moving Northward at the march equinox and crosses the equator moving Southward at the September equinox.

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