Category: Class 11 Physics Objective Questions

Physics Multiple Choice Questions on “System of Particles – Motion of Centre of Mass”.

1. If forces are acting on a rigid body so that it has zero kinetic energy, then all forces will pass through the centre of mass and have a vector sum of zero. True or False?
a) True
b) False
Answer: b
Clarification: If a rigid body has zero kinetic energy means that vector sum of all forces will be zero and the torque on it will be zero. For torque to be zero the vector cross product of force and distance should be zero. It is not necessary for all forces to pass through the centre.

2. In a projectile motion, a particle breaks due to internal forces. We can say that its total momentum is conserved. True or False?
a) True
b) False
Answer: b
Clarification: For momentum to be conserved its value should remain constant, but in projectile motion the force of gravity is always changing its momentum. The internal forces will break the particle and the momentum of different particles can be conserved in a direction perpendicular to direction of gravity.

3. A ball of mass 1kg is thrown vertically upwards from a building with a speed of 10m/s, and another ball of mass 3kg is dropped from the same point towards the ground with the same speed. At what time will the centre of mass have the maximum height w.r.t the ground? Assume balls are left at t=0.
a) 0s
b) 1s
c) 2s
d) 2.8s
Answer: a
Clarification: When the balls are released in their respective directions, the ball going downwards is heavier and is covering more distance per unit time. This is because both the balls have the same initial speed and the heavier ball is gaining speed downwards while the other ball is losing speed as it moves upwards. Their mid-point will be below the point of release and centre of mass, will therefore, be even below that point, since heavier mass is moving downwards.

4. A boy of mass 50kg is standing on a frictionless surface. He throws a ball of mass 2kg away from him with a speed of 10m/s. Find the final speed of the centre of mass.
a) 0m/s
b) 20m/s
c) 10m/s
d) 0.4m/s
Answer: a
Clarification: Initially the boy was at rest and there is no friction, so the net force on the ball-boy system is zero. Hence, their momentum change will be zero and final velocity of centre will also be zero. Note that boy will move backwards with a speed of 2*10/50 = 0.4m/s.

5. A ball of mass 3kg is thrown at an angle of 30° with the horizontal & a speed of 10m/s. At the highest point the ball breaks into two parts, having mass ratio 2:1, due to internal forces. If the heavier part falls at a distance of 10m from the start, find the x coordinate of the second part.
a) 5.98m
b) 6.34m
c) 8.66m
d) 7.99m
Answer: a
Clarification: There is no net force on the particle in the x-direction, so the x-coordinate of centre of mass will be equal to the range of projectile motion.
The parts will have masses 2kg and 1kg.
The time of motion ‘t’ = 2u_y/g = 1s.
The range = u_xt = 5√3*1 = 8.66m.
The centre of mass will land at 8.66m.
So,
(m₁x₁ + m₂x₂)/(m₁ + m₂) = 8.66.
m₁ = 2kg, m₂ = 1kg, x₁ = 10m, we have to calculate x₂.
∴ m₂x₂ = (8.66*3) – 2*10
= 25.98 – 20 = 5.98.
∴ x₂= 5.98/1 = 5.98m.

Physics Multiple Choice Questions on “Gravitation – Earth Satellite – 1”.

1. The satellites orbiting the earth, eventually fall to the earth when they are left unsupervised or unattended because _____
a) their power supply runs out
b) of viscous forces causing the speed of the satellite and hence height to gradually decrease
c) the laws of gravitation predict such a trajectory
d) of collisions with other satellites

Answer: b
Clarification: Due to the presence of some gases of the atmosphere and other debris, the satellites speed is reduced due to viscous forces. This reduces the height of the satellite. Hence, it eventually falls to the earth.

2. The time period of the moon around the earth is 24 hours.
a) True
b) False

Answer: b
Clarification: The time period of the moon around the earth cannot be 24 hours because then it would be a geostationary satellite. However, we know it is not true. The time period of the moon around the earth is found to be approximately 27.3 earth days.

3. Consider 2 satellites A and B having time periods 16 hours and 1 hour, respectively. What is the ratio of the radius of their orbits?
a) 4:1
b) 8:1
c) 16:1
d) 64:1

Answer: d
Clarification: The time periods (T) of a satellite revolving around the earth of radius R and at height h is:
T = 2 x pi x [(R + h)³ / (G X M)]^1/2
Hence, the time period is directly proportional to (R + h)^3/2
The ratio of orbits is, therefore, 64:1.

4. A satellite is launched into a circular orbit of radius R while a second satellite is launched into an orbit of radius 1.02R. What is the percentage change in the time periods of the two satellites?
a) 0.7
b) 1
c) 1.5
d) 3

Answer: d
Clarification: The time periods (T) of a satellite revolving around the earth of radius R and at height h is:
T = 2 x pi x [(R + h)³ / (G X M)]^1/2
Hence, the time period is directly proportional to (R + h)^3/2
From this, we have the ratio of the time periods as 1.03.
1.03 x 100 = 103. Hence, there is a 3% change in the time period.

5. A satellite is revolving very close to a planet of density D. What is the time period of that satellite?
a) [3/(D*G)]^1/2
b) [3/(D*G)]^3/2
c) [3/(2*D*G)]^1/2
d) [(3*G)/D]^1/2

Answer: c
Clarification: The time period of a satellite flying very close to the surface of the earth is;
T = 2 x pi x [R³ / (G X M)]^1/2;  [Height is negligible compared to the radius of the earth].
Mass (M) = Density (D) x Volume
M = D x (4/3 x pi x R³)
Substituting the relation of mass into the time period, we get;
T = [3/(2 x D x G)]^1/2

6. A satellite orbits the earth at a height of R/5. What is its orbital speed?
a) [(2*G*M)/(R)]^1/2
b) [(G*M)/(R)]^1/2
c) [(G*M)/(7*R)]^1/2
d) [(5*G*M)/(6*R)]^1/2

Answer: d
Clarification: The orbital velocity of a satellite flying at a height “h” above the surface of the earth of radius “R” is:
v = [(G*M)/(R+h)]^1/2
Here, h = R/5
Therefore; v = [(5*G*M)/(6*R)]^1/2

7. The time period of a satellite is independent of the mass of the satellite.
a) True
b) False

Answer: a
Clarification: For a satellite of mass “m” revolving around a planet of mass “M” and radius
“R” at a height “h” above the surface, the satellite traverses a distance [2 x pi x (R + h)] in time “T” if the orbital velocity is “v”.
Hence, the time periods (T) = [2 x pi x (R + h)]/v
We know; orbital velocity (v) = [(G x M)/(R+h)]^1/2
Hence; T = [2 x pi x (R + h)^3/2/(G X M)^½], which is independent of the mass of the satellite.

8. The time period of a satellite of earth is 90 minutes. If the separation between the earth and the satellite is quadrupled, the new time period will be _____________
a) 90 minutes
b) 180 minutes
c) 560 minutes
d) 720 minutes

Answer: d
Clarification: The time period is directly proportional to the radius of orbit raised to the power of 1.5.
If the radius is quadrupled, the time period increases by 8 times.
Hence, the new time period = 90 x 8 = 720 minutes.

Physics Multiple Choice Questions on “Temperature and Heat”.

1. An aluminium piece and a wooden piece have been left in a room for a few hours. When you go and touch them the aluminium seems colder than wood. Select the correct option. Assume room temperature to be 25°.
a) Both have the same temperature greater than 25°
b) Aluminium has a greater temperature than wooden piece
c) Aluminium has a lower temperature than wooden piece
d) Both have the same temperature equal to 25°
Answer: d
Clarification: Both have been left in a room for a few hours, so both will have the same temperature as that of the room. The only reason aluminium feels colder is because it conducts heat away from our body faster than wood.

2. An ice cube is kept on the surface of an aluminium block and another ice cube on the surface of a plastic block. Both blocks have the same temperature. Select the correct statement.
a) Ice will melt faster on the aluminium block
b) Ice will melt faster on the plastic block
c) Ice will melt equally fast on both the blocks
d) Ice will not melt on the plastic block
Answer: a
Clarification: Ice will melt faster on the aluminium block because it has a higher thermal conductivity than plastic and hence conducts heat more rapidly than plastic. Which is why ice melts faster on aluminium.

3. When an object is heated its temperature definitely increases?
a) True
b) False
Answer: b
Clarification: When we supply heat to a body it’s not necessary for its temperature to increase. The heat may increase temperature, it may expand or change state. For eg: when ice is melting, we provide heat but conversion from ice to water takes place at constant temperature.

Physics Multiple Choice Questions on “Thermodynamics – Refrigerators and Heat Pumps”.

1. What is the coefficient of performance of a refrigerator? Let Q₁ be the heat released to hot reservoir, Q₂ be the heat extracted from a cold reservoir & W be the work done on the refrigerator.
a) Q₁/W
b) Q₁/Q₂
c) Q₂/W
d) Q₂/Q₁
Answer: c
Clarification: Coefficient of performance of a refrigerator is defined as the ratio of heat extracted from the cold reservoir to the work done on the refrigerator. It is denoted by α = Q₂/W.

2. Select the correct order of steps in which a working substance(gas) goes through in a refrigerator.
a) Expansion of gas, absorption of heat, heating of vapour, release of heat
b) Absorption of heat, expansion of gas, heating of vapour, release of heat
c) Heating of vapour, expansion of gas, absorption of heat, release of heat
d) Heating of vapour, absorption of heat, expansion of gas, release of heat
Answer: a
Clarification: In a refrigerator, the working substance(gas) first gets expanded from high to low pressure which cools it into a vapour-liquid mixture. Then heat is absorbed from the region to be cooled. Then the external work done on the system heats up the vapour. Finally, the vapour’s heat is released to the surroundings.

3. Efficiency of a refrigerator can be greater than 1. True or False?
a) True
b) False
Answer: b
Clarification: Efficiency by definition can never be greater than 1. Else the law of energy conservation will be violated.

4. The coefficient of a refrigerator can be greater than 1. True or False?
a) True
b) False
Answer: a
Clarification: The coefficient of a refrigerator is defined as α = Q₂/W,
where Q₂ is heat extracted from a cold reservoir and W is work done.
α = Q₂/( Q₁ – Q₂).
Therefore α can be less than or greater than one.

5. 20J of heat energy is extracted from a cold reservoir when 30J of work is done on a refrigerator. What is the coefficient of the refrigerator?
a) 2/3
b) 5/3
c) 3/5
d) 1/3
Answer: a
Clarification: The coefficient of a refrigerator is defined as α = Q₂/W,
where Q₂ is heat extracted from a cold reservoir and W is work done.
α = 20/30 = 2/3.