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Physics Chapter 1 : Topic 3
Formula Derivation by Dimensional Analysis
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A simple pendulum has time period ‘$\mathbf{T}$’ depending on length ‘$\mathbf{l}$’ and gravitational acceleration ‘$\mathbf{g}$’. Using dimensional analysis, the form of ‘$\mathbf{T}$’ is:
Reasoning: Dimensional analysis gives $\ \mathbf{T = k}$ $\sqrt{\frac{\mathbf{l}}{\mathbf{g}}}\mathbf{\ }$, where $\mathbf{k}$ is dimensionless.
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If $\mathbf{T \propto}\mathbf{l}^{\mathbf{a\ }}\mathbf{g}^{\mathbf{b}}$, then the value of ‘$\mathbf{a}$’ for a simple pendulum is:
Reasoning: Solving dimensions gives $\mathbf{a}\mathbf{=}$ $\frac{\mathbf{1}}{\mathbf{2}}$.
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The time period of a pendulum is independent of:
Reasoning: Dimensional analysis shows mass does not appear in the relation.
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Escape velocity ‘$\mathbf{v}$’ depends on gravitational acceleration ‘$\mathbf{g}$’ and radius ‘$\mathbf{r}$’ of a planet. The correct dimensional formula is:
Reasoning: Dimensions give $\mathbf{v = (gr}\mathbf{)}^{\mathbf{1/2}}$.
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The dimensions of $\mathbf{gr}$ are:
Reasoning: $\mathbf{g =}\left\lbrack \mathbf{L\ }\mathbf{T}^{\mathbf{- 2}} \right\rbrack\mathbf{\ }$, $\mathbf{r = \lbrack L\rbrack}$
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The speed of waves on a stretched string depends upon tension ‘$\mathbf{F}$’ and mass per unit length ‘$\mathbf{\mu}$’. The correct relation is:
Reasoning: Dimensional analysis yields $\mathbf{v = (F\ /\ \mu}\mathbf{)}^{\mathbf{1/2}}$.
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If $\mathbf{v \propto}\mathbf{F}^{\mathbf{a\ }}\mathbf{\mu}^{\mathbf{b}}$, the value of ‘$\mathbf{a}$’ is:
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The dimensions of mass per unit length are:
Reasoning: $\mathbf{\mu = M\ /\ L}$
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The kinetic energy ‘$\mathbf{E}$’ of a body depends on mass ‘$\mathbf{m}$’ and velocity ‘$\mathbf{v}$’. Dimensional analysis predicts:
Reasoning: Dimensions require $\mathbf{E \propto m}\mathbf{v}^{\mathbf{2}}$.
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Dimensional analysis cannot determine:
Reasoning: Constants like $\frac{\mathbf{1}}{\mathbf{2}}$, $\mathbf{2\pi}$ cannot be obtained.
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The force ‘$\mathbf{F}$’ acting on a body depends on mass ‘$\mathbf{m}$’ and acceleration ‘$\mathbf{a}$’. Dimensional analysis gives:
Reasoning: Dimensions match only for $\mathbf{ma}$.
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In dimensional analysis, if the number of unknown exponents exceeds the number of fundamental dimensions, then:
Reasoning: There may be insufficient equations to determine all powers.
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If pressure ‘$\mathbf{P}$’ depends on force ‘$\mathbf{F}$’ and area ‘$\mathbf{A}$’, then:
Reasoning: Pressure is force per unit area.
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Which quantity is commonly expressed as $\mathbf{v \propto}\sqrt{\mathbf{gr}}\mathbf{\ }$?
Reasoning: Escape velocity follows this dimensional form.
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If a formula derived dimensionally is $\mathbf{Q \propto}\mathbf{a}^{\mathbf{2}}\mathbf{\ }\mathbf{b}^{\mathbf{- 1}}$, then increasing $\mathbf{a}$ by $\mathbf{2}$ times changes $\mathbf{Q}$ by:
Reasoning: $\mathbf{Q \propto}\mathbf{a}^{\mathbf{2}}$
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The dimensions of energy are:
Reasoning: Energy $\mathbf{=}$ force $\mathbf{\times \ }$distance
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A student derives $\mathbf{T = k}$ $\sqrt{\frac{\mathbf{l}}{\mathbf{g}}}$. The symbol $\mathbf{k}$ represents:
Reasoning: Dimensional analysis cannot determine numerical constants.
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The speed of a wave depends on tension ‘$\mathbf{F}$’ and linear density ‘$\mathbf{\mu}$’. If tension becomes four times, speed becomes:
Reasoning: $\mathbf{v \propto}\sqrt{\mathbf{F}}$
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A formula derived dimensionally is most useful for:
Reasoning: It reveals how quantities depend on each other.
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If gravitational acceleration becomes four times, the period of a pendulum becomes:
Reasoning: $\mathbf{T \propto 1\ /\ }\sqrt{\mathbf{g}}$
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Which quantity has dimensions $\mathbf{\lbrack}\mathbf{M\ L}^{\mathbf{- 1}}\mathbf{\rbrack}$?
Reasoning: Linear density $\mathbf{=}$ mass / length
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If length of a pendulum becomes four times, its period becomes:
Reasoning: $\mathbf{T \propto}\sqrt{\mathbf{l}}$
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Which of the following can be derived using dimensional analysis?
Reasoning: Dimensional analysis determines proportionality, not numerical constants.
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If escape velocity $\mathbf{v \propto}\sqrt{\mathbf{gr}}$, doubling radius while keeping$\mathbf{g}$ constant makes $\mathbf{v}$:
Reasoning: $\mathbf{v \propto}\sqrt{\mathbf{r}}$
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If $\mathbf{Q \propto}\mathbf{a}^{\mathbf{x}}\mathbf{\ }\mathbf{b}^{\mathbf{y}}$, dimensions are compared to:
Reasoning: Comparing dimensions gives equations for exponents.
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Why is dimensional analysis called a powerful tool?
Reasoning: It provides useful relations before experimentation.
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Which step comes first in dimensional derivation?
Reasoning: Begin by assuming a relation containing unknown powers.
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Which formula cannot be completely obtained by dimensional analysis?
Reasoning: The factor $\mathbf{2\pi}$ cannot be derived dimensionally.
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The dimensions of $\mathbf{F\ /\ \mu}$ are:
Reasoning: $\left\lbrack \mathbf{M\ L\ }\mathbf{T}^{\mathbf{- 2}} \right\rbrack\mathbf{\ \ /\ \ }\left\lbrack \mathbf{M\ }\mathbf{L}^{\mathbf{- 1}} \right\rbrack\mathbf{=}\left\lbrack \mathbf{L}^{\mathbf{2\ }}\mathbf{T}^{\mathbf{- 2}} \right\rbrack$
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Taking square root of $\mathbf{F\ /\ \mu}$ gives dimensions of:
Reasoning: $\left\lbrack \mathbf{L}^{\mathbf{2\ }}\mathbf{T}^{\mathbf{- 2}} \right\rbrack^{\mathbf{1/2}}\mathbf{=}\left\lbrack \mathbf{L\ }\mathbf{T}^{\mathbf{- 1}} \right\rbrack$
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Which of the following is a limitation of dimensional analysis?
Reasoning: Dimensional analysis cannot reveal whether terms are added or subtracted.
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If kinetic energy depends on mass and velocity only, dimensional analysis predicts the power of velocity to be:
Reasoning: $\mathbf{E \propto m}\mathbf{v}^{\mathbf{2}}$
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The dimensions of $\mathbf{g\ /\ }\mathbf{l}$ are:
Reasoning: $\left\lbrack \mathbf{L\ }\mathbf{T}^{\mathbf{- 2}} \right\rbrack\mathbf{\ \ /\ \ \lbrack L\rbrack =}\left\lbrack \mathbf{T}^{\mathbf{- 2}} \right\rbrack$
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The square root of $\mathbf{g\ /\ }\mathbf{l}$ has dimensions:
Reasoning: $\sqrt{\mathbf{T}^{\mathbf{- 2}}}\mathbf{=}\mathbf{T}^{\mathbf{- 1}}$
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A correct dimensional derivation requires:
Reasoning: Dimensional equations are solved by equating dimensions on both sides.
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