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ALFA PHYSICS CLASSES
LEAD. INNOVATE. INSPIRE
Q.1
A.1
Q.2
A.2
The bob of simple pendulum is negatively charged and a positively charged plate is just
placed below the bob. What will the effect on the time period of oscillation?
As there will be attractive force between opposite charges in downward direction, the
effective value of g for the pendulum bob will increase thus decreasing the time period of
oscillation of the bob.
A girl is sitting on a swing. Another girl sits by her side. What will be the affects on the
periodic time of the swing?
There will be no effect on the time period of oscillation. The time period of swing depends on
the length of the swing and the acceleration due to gravity at the place. If another girls sits
only mass of the swing changes and time period doesn’t depend on mass.
Q.3
A.3
The girl sitting on a swing stands up. How will the time period be affected?
If the girl stands up the distance between point of suspension and center of mass of the girl
will decrease thus decreasing the length of the pendulum. Thus, time period of the swing will
also decrease.
Q.4
What provides restoring force for SHM in [a] pendulum [b] spring pendulum and [c] liquid
oscillating in U tube?
In Simple pendulum gravity provides the restoring force where as in spring pendulum the
elasticity of the spring provides the restoring force. For U tube the weight of the liquid
provides restoring force.
A.4
Q.5
A.5
Q.6
A.6
Q.7
If pendulum clock is taken to mountain top does it loose or gain time, assuming it gives
correct time at lower elevation?
As we move above the surface of earth the acceleration due to gravity is going to decrease
thus the time period of oscillation of the pendulum will increase. Thus the clock will go slow
on the mountain top and it looses time.
The amplitude of simple harmonic oscillator is doubled how will it affect the [a] the maximum
velocity [b] the total energy [c] the period of oscillation?
The maximum velocity V = r, thus if the amplitude is doubled the maximum velocity
becomes double. The total energy is directly proportional to the square of amplitude of the
wave. Thus, if amplitude is doubled the total energy becomes four times. The time period is
independent of the amplitude of motion and remains unchanged.
At what distance from the mean position the kinetic and potential energy of simple harmonic
oscillator equal?
A.7
Kinetic energy = potential energy


1
1
m 2 r 2  y 2  m 2 y 2
2
2
Q.8
A.8
Q.9
A.9
y= r/2
For an oscillating pendulum, is tension in the string constant throughout? If not, where it is
[a] minimum and [b] maximum?
tension in the string changes with the change in angle which the pendulum string makes with
the vertical. It is maximum at the lowest point and minimum at the extreme position.
The soldiers are told to break their steps while marching on the bridge. Why?
Soldiers are told to break their steps because if the frequency of marching soldiers becomes
equal to the natural frequency of the bridge, resonance will occur which results in increased
amplitude of oscillation for the bridge and it may break.
Residence: 249, Chotti Baradari Part –2,[Near Medical College], Garha Road, Jalandhar #98152-15362
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ALFA PHYSICS CLASSES
LEAD. INNOVATE. INSPIRE
Q.10
A.10
Q.11
A.11
Why amplitude of vibrating pendulum should be small?
If the amplitude of vibration is small we can make an assumption that mg sin= mg, but if
the amplitude of oscillation is large we can’t make this assumption thus the restoring
acceleration will not be proportional to the displacement and motion will not be SHM.
What is necessary and sufficient condition for motion to be simple harmonic?
The necessary and sufficient condition for motion to be SHM is that the restoring force is
always proportional to the displacement and the direction of restring force is opposite to
displacement.
Q.12
A.12
How will the time period of simple pendulum change when its length is doubled/
The time period of simple pendulum is directly proportional to the square root of the length of
pendulum, thus if length of the pendulum is doubled the time period will increase 2 times.
Q.13
When a body of mass 2kg is suspended by a spring the spring is stretched. If th ebody is
pulled down and released, it oscillates up and down. What force should be applied on the
body by the spring when it passes through the mean position?
When body passes through the mean position, the weight of the body acting downward is
balanced by upward restoring force in the spring. Thus net force as well as the acceleration at
the mean position is zero.
A.13
Q.14
A.14
Can motion of the body be oscillatory but not SHM? If yes, give an example.
Yes, for motion to be SHM restoring acceleration magnitude is always proportional to the
displacement which is not always the case in oscillatory motion. For e.g. if ball is dropped on
horizontal elastic surface it oscillates up and down but its not SHM as constant value of g acts
on it.
Q.15
A.15
Will the time period of spring pendulum change if it is taken to moon?
The time period of spring pendulum depends on the mass of the body attached and spring
constant of the spring. Both the values will remain same on moon as on earth thus time
period will remain unchanged.
Q.16
A vibrating simple pendulum of time period T is a placed in a lift, which is accelerating
downwards. What will be the effect on the time period?
If the lift accelerates downward, pseudo force acts on the body in upward direction. Thus the
value of acceleration becomes (g-a) and the time period is going to increase as effective
value of g decreases.
A.16
Q.17
A.17
Can the motion of artificial satellite around earth be taken as SHM?
No because it is periodic motion and not oscillatory motion. For SHM body should move to
and fro about the mean position whereas the satellite just orbits around earth.
Q.18
A.18
What are the factors on which the natural frequency of the body depends?
Natural frequency of body depends upon [a] elastic properties of the materials of the body
and [b] dimensions of the body.
Q.19
A.19
Is the damping forces acting on the body in SHM constant?
No the damping forces are dependent on the velocity of the body therefore they can’t be
constant as in SHM velocity keeps on changing with time.
Q.20
When a pendulum clock gains time, what adjustment should be made?
Residence: 249, Chotti Baradari Part –2,[Near Medical College], Garha Road, Jalandhar #98152-15362
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A.20
If the pendulum cock gains time it means that it has gone fast or the time period of
oscillations has decreased. Thus, for correct time the length of the pendulum should be
increased accordingly.
Q.21
The bob of simple pendulum is made of ice. How will the time period change when the ice
melts?
If the ice melts, the time period will remain unchanged because the distance between point of
suspension and center of the bob remains unchanged. But if the center of gravity of ice starts
moving upwards due to melting of ice then the time period will decrease.
A.21
Q.22
A.22
Q.23
A.23
Q.24
A.24
Q.25
A.25
What would happen to the motion of oscillating system if the sign of force term in equation
F=-kx is changed?
If we change the sign equation will be F=kx which implies that force will be in the direction of
the displacement. Thus motion will not be SHM but it will be linearly accelerated motion.
The bob of simple pendulum is a ball full of water. If a fine hole is made at the bottom so that
water slowly leaks, what will be the effect on the time period of the pendulum?
If a small hole is made at the bottom of the ball, and water starts coming out the center of
mass of the pendulum moves down. Thus the effective length of the pendulum increases and
time period also goes on increasing. But when the ball is completely empty the center of
mass again shifts to initial value and time period will also return to its initial lesser value.
Two exactly identical pendulums are oscillating with amplitudes 2cm and 4cm. Find the ratio
of their energies of oscillation?
Energy of oscillation is directly proportional to the square of amplitude. Thus, the ratio of
square of amplitudes is ratio of energies i.e. 1:4
A ball of radius r is made to oscillate in bowl of radius R, find its time period of oscillation.
Here, the equivalent length of simple pendulum= distance between center of ball to center of
bowl i.e. l=(R-r). Time period of oscillation of ball = 2
Q.26
A.26
Q.27
A.27
A pendulum clock is placed on the moon, where object weighs only one sixth as much as on
earth. How many times the clock tick out in an actual time of 1 minute, the clock keeps good
time on earth.
If the time period on earth is 1 second the time period on moon will be 6 seconds. Thus, the
number of times clock tick in a day is 60/6
What will happen to the time period of simple pendulum if its amplitude of oscillation is large?
If amplitude of oscillation is large the restoring force is not proportional to the displacement.
If  is angular amplitude of oscillation then
T = 2
Q.28
A.28
(R  r ) / g
l
1


1  2 sin 2  .......

g 2
2

The maximum acceleration of a simple pendulum is a while maximum velocity is v. what is
the displacement amplitude?
The maximum acceleration of the pendulum is a=r2 and maximum velocity is v=r. From
these two equations we can say that r = v2/a
Residence: 249, Chotti Baradari Part –2,[Near Medical College], Garha Road, Jalandhar #98152-15362
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