Motion Under Gravity

Total Questions: 41

31. If a person sitting in a lift then when will he feel that his weight gets increased ? [U.P.P.C.S.(pre) 1990]

(a) when the elevator going upwards expeditously
(b) when the elevator going down expeditously
(c) going upwards with constant velocity
(d) going downwards with constant velocity

Correct Answer: (a) when the elevator going upwards expeditously

Solution:√Let the mass of the person is m and weight is w and the elevator going upwards with acceleration a. the force acting on person
1.the force due to gravity =mg
2. the force giving the acceleration =ma
according to newton second law -
w-mg=ma
w=m(g+a)
so it is clear that when the elevator is going upwards a person actually feels a little heavier then the usual weight and on the other hand when the elevator accelerate downward then a person feels a little reduction than his usual weight.

32. The time period of a pendulum - [67th B.P.S.C. (PRE) 2005]

(a) Depends on the mass
(b) depends on the length
(c) Depends on the time
(d) Depends on the temperature

Correct Answer: (b) depends on the length

Solution:The time period of a pendulum depends on its length. as we know that T=2π√l/g thus the result is that the one variable that effect the period of the pendulum is the length of the string.Increase in the length lead to increase in the period.

33. Why do pendulum clocks usually lose time in summer ? [U.P.P.C.S.(pre) 2012, 1994]

(a) due to longer days in summer
(b) due to friction caused by coils
(c) The length of the pendulum increase increasing the time period of unit oscillation.
(d) the weight of the pendulum change in summer

Correct Answer: (c) The length of the pendulum increase increasing the time period of unit oscillation.

Solution:time period of simple pendulum T=2π√l/g
where l=length of the pendulum
in summer the length of the pendulum is slightly increased due to this time period also increased.This results that the pendulum clocks are slow in summer.

34. A girl is swinging on a swing in sitting position.When the girl stand up the period of the swing will : [I.A.S. (PRE) 1997]

(a) be shorter
(b) be longer
(c) depends on the height of the girl
(d) not change

Correct Answer: (a) be shorter

Solution:A girl is swinging on a swing position but when she stands up she is effectively moving her mass higher thus shortening the length of the pendulum.shortening this lengths will decrease the period or the period of the swing will be shorter.

35. consider the following statement :A simple pendulum is set into the oscillation then : [I.A.S. (PRE) 2001]

1.the acceleration is zero when the bob passes through the mean position

2.in each cycle the bob attains a given velocity twice .

3.both the acceleration and the velocity of the bob are zero when its reaches its extreme position during the oscillation.

4.the amplitude of oscillation of the simple pendulum decrease with time.

which of the following are correct ?

(a) 1 and 2
(b) 3 and 4
(c) 1, 2, and 4
(d) 2 , 3, and 4

Correct Answer: (c) 1, 2, and 4

Solution:When an object, which is in simple periodic motion passes through its mean position then,
(i) No force acts on it.
(ii) It's acceleration becomes zero.
(iii) Maximum velocity.
(iv) Maximum kinetic energy.
(v) Potential energy is zero.
When the object reaches its extreme position then,
(i) Its acceleration is maximum.
(ii) Counterforce acting on that is maximum.
(iii) It's Kinetic energy is zero.
(iv) Has maximum potential energy.
(v) Has zero velocity.
In each rotation, the pendulum twice attains a specific velocity. In addition, under normal circumstances oscillation of usual pendulum decreases with time (due to air resistance etc.). Thus the statement 1, 2 and 4 are correct while statement 3 is wrong.

36. A pendulum clock can run faster in - [R.A.S./R.T.S (PRE) 1997]

(a) summer
(b) winter
(c) spring season
(d) rainy season

Correct Answer: (b) winter

Solution:A pendulum clocks run faster in winter because in winter the length of the pendulum or the swing become smaller due to contraction and the clock begin to run fast.

37. Earth's escape velocity is - [R.A.S./R.T.S (PRE) 1993]

(a) 15.0 km/sec
(b) 21.1 km/sec
(c) 7.0 km/sec
(d) 11.2 km/sec

Correct Answer: (d) 11.2 km/sec

Solution:The term escape velocity means the minimum speed that a moving object must have to escape from the gravitational field of a celestial body and move outward into the space. The escape velocity of the earth is 11.2 km/sec.

38. What will happen if an object is thrown into the space from the the speed of 8 km/sec ? [R.A.S./R.T.S (PRE) 1992]

(a) the object will go into the space
(b) it will return to earth
(c) it will rotate into the orbit of earth
(d) it will burst out

Correct Answer: (b) it will return to earth

39. There is no atmosphere on the moon because - [U.P.P.C.S.(MAINS) 2012]

(a) it is near to the earth
(b) it receives light from the sun
(c) it revolves around the sun
(d) the escape velocity of the gas molecules on it is less than the root mean square velocity here.

Correct Answer: (d) the escape velocity of the gas molecules on it is less than the root mean square velocity here.

Solution:Moon has no atmosphere because the value of the acceleration due to gravity (g) on the surface of the moon is small.Therefore the value of escape velocity on the surface of the moon is small.The value of the root mean square velocity of the molecules of different gases is much above the value of the escape velocity on the moon.That is why all the molecules of gases escaped and there is no atmosphere on the moon.

40. Given below are two statement - [U.P.P.C.S.(pre) 2009]

Assertion (A) : Lighting a candle in space does not produce a flame.
Reason (R) : A flame exists due to the pull of gravity in the context of above which one of the following is correct ?

(a) both (A) and (R) are true and (R) is the correct explanation of (A)
(b) both (A) and (R) are true and (R) is not the correct explanation
(c) (A) is true but (R) is false
(d) (A) is false but (R) is true

Correct Answer: (a) both (A) and (R) are true and (R) is the correct explanation of (A)

Solution:NASA is made it clear by his experiment that gravitation is must for lighting up a candle .In the case of microgravity in the spacecraft we are able to light up the flame but it is not possible in an actual vaccum.In fact the flickering frequency of a flame is proportional to the square root of the acceleration due to gravity to the diameter of the candle and due to the lack of gravitational force in space it is zero.Thus the correct answer is (a)

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Motion Under Gravity