Motion Under Gravity (Physics)

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]

Correct Answer: (a) When the elevator going upwards expeditiously.
Solution:

Let the mass of the person is 'm' and weight is 'w' and the elevator going upward 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's second law -

w - mg = ma

w = m(g + a)

So, it is clear that when the elevator is going upward expeditiously a person actually feels a little heavier than his usual weight and on the other hand when elevator accelerates 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]

Correct Answer: (b) Depends on its length
Solution:

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

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

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

Correct Answer: (a) Be shorter
Solution:

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

35. Consider the following statement : [I.A.S. (Pre) 2001]

 

A simple pendulum is set into the oscillation then :

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

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

Which of these statements are correct?

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]

Correct Answer: (b) Winter
Solution:

A pendulum clock runs 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]

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 space. The escape velocity of 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]

Correct Answer: (b) it will return to earth
Solution:

11.2 km/sec is the escape velocity of the Earth which is minimum speed that is required to escape from the Earth's gravity. If an object thrown with the speed less than this, it will return to Earth.

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

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 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 root mean square velocity of the molecules of different gases is much above the value of 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?

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

NASA 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 vacuum. 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).