# Engineering Physics Questions and Answers – Relation between Torque and Momentum

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Relation between Torque and Momentum”.

1. If no external torque acts on a body, its angular velocity remains conserved.
a) True
b) False

Explanation: If no external torque acts on a body, its angular velocity is not conserved but angular momentum is conserved.

2. Which component of linear momentum does not contribute to angular momentum?
a) Horizontal component
b) Vertical component
d) Tangential component

Explanation: The radial component does not contribute to the angular momentum.

3. A heavenly body revolves around a massive star in a highly elliptical orbit. Ignoring mass loss of the heavenly body when it comes too close to the star, the angular momentum over the orbit is _____________
a) Zero
b) Increasing
c) Decreasing
d) Constant

Explanation: The heavenly body revolves around the massive star under the effect of gravitational force, which is purely radial. The torque exerted by such a force is zero. Hence the angular momentum of the heavenly body remains constant over its entire orbit.

4. A body can be in equilibrium while it is in motion.
a) True
b) False

Explanation: A body can be in equilibrium while it is in motion. A body in motion will be in equilibrium if it has no linear and angular accelerations. Hence a body moving with uniform velocity along a straight line will be in equilibrium.

5. We can rise from a chair without bending a little forward.
a) True
b) False

Explanation: Our weight exerts a torque about our feet. This makes it difficult for us to rise from the chair. When we bend forward, the centre of gravity of our body comes above our feet. The torque due to our weight becomes zero and we can easily rise from the chair.
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6. When a cat falls from a height _____________
a) It sustains injury
b) It dies
c) It lands on its feet
d) It bounces back

Explanation: When a cat falls to the ground from a height, it stretches its body along with the tail so that its moment of inertia becomes high. The value of angular speed decreases and therefore the cat is able to land on the ground gently.

7. How many propellers are there in a helicopter?
a) One
b) No propellers
c) Three
d) Two

Explanation: If there were only one propeller in the helicopter then, due to the conservation of angular momentum, the helicopter itself would have turned in the opposite direction.

8. If the earth were to shrink suddenly, what would happen to the length of the day?
a) It increases
b) It remains the same
c) It decreases
d) It becomes unpredictable

Explanation: When the earth shrinks, the moment of inertia (I=2/5 MR2) decreases about its own axis due to the decrease in radius R. To converse angular momentum, the time period T decreases. That is, the length of the day decreases.

9. A child is sitting on a swing. Its minimum and maximum heights from the ground 0.75m and 2m respectively, its maximum speed will be _____________
a) 10 m/s
b) 5 m/s
c) 8 m/s
d) 15 m/s

Explanation: Maximum kinetic energy = Drop in potential energy
1/2 mv2=mg(h2-h1)
v=√(2g(h2-h1))=√(2×10(2-0.75))=5m/s.

10. A moving body of mass m and velocity 3km/h collides with a rest body of mass 2m and strikes to it. Now the combined mass starts to move. What will be the combined velocity?
a) 3km/h
b) 4km/h
c) 1km/h
d) 2km/h

Explanation: By conservation of momentum,
m×3+2m×0=(m+2m)v
v = 1km/h.

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