Consider the following statements: 1. There is no net moment on a body which is in equilibrium. 2. The momentum of a body is always conserved. 3. The kinetic energy of an object is always conserved. Which of the statements given above is/are correct?
- A.1, 2 and 3
- B.2 and 3 only
- C.1 and 2 only
- D.1 only
Correct answer
Explanation
The correct answer is D, 1 only. A body in equilibrium has no net force and no net moment on it, while neither momentum nor kinetic energy is conserved in every situation.
For a rigid body to be in equilibrium, two conditions must hold: the vector sum of forces is zero and the sum of moments (torques) about any point is zero. Momentum is conserved only for a system on which no net external force acts; a ball pushed by a hand gains momentum. Kinetic energy is conserved only in perfectly elastic collisions; in an inelastic collision part of it turns into heat and sound, even though total energy is conserved.
A is wrong: statements 2 and 3 are not always true.
B is wrong: it drops the correct statement 1 and keeps the two wrong ones.
C is wrong: statement 2 fails whenever an external force acts.
Exam tip: momentum conserved when external force is zero; kinetic energy conserved only in elastic collisions.
Practice Questions
View allWhich of the following properties is true for a tooth paste?
- A.It is acidic
- B.It is neutral
- C.It is basic
- D.It is made up of Calcium phosphate, the material of tooth enamel
Show answer
Toothpaste is mildly basic so that it can neutralise the acid that causes tooth decay. Bacteria in the mouth break down sugar and food particles left on the teeth and produce acids.
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- A.P' = P
- B.P' = 2P
- C.P' = P/2
- D.P' = P/4
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Read the full explanation →In which of the following pairs are the ions isoelectronic?
- A.Mg²⁺, Ar
- B.Na⁺, O²⁻
- C.Al³⁺, Cl⁻
- D.K⁺, Ne
Show answer
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- A.The disc
- B.The sphere
- C.Both have the same moment of inertia
- D.The information provided is not sufficient to answer the question
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Read the full explanation →Numerically two thermometers, one in Fahrenheit scale and another in Celsius scale shall read same at
- A.−40°
- B.0°
- C.−273°
- D.100°
Show answer
At this one temperature the Celsius and Fahrenheit scales show the same number. The two scales are linked by F = (9/5)C + 32.
Read the full explanation →