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Chapter 3: Problem 19
The average speed \(=\) uniform speed \(=10 \mathrm{~km} \mathrm{~h}^{-1}\)
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Speed \(=\frac{\text { Distance travelled }}{\text { Time taken }}\) \(\Rightarrow\) distance travelled by the body \(=\) speed of the body \(\times\) time taken.
From the given figure, by pythogorous theorem \(\mathrm{PR}^{2}=\mathrm{PQ}^{2}+\mathrm{R} \mathrm{Q}^{2}\) \(\mathrm{PR}^{2}=4^{2}+3^{2}\) \(\mathrm{PR}^{2}=16+9\) \(\mathrm{PR}=\sqrt{25}\) \(\mathrm{PR}=5 \mathrm{~m}\) Average speed \(=\frac{\text { total distance }}{\text { total time }}=\frac{4 \mathrm{~m}+3 \mathrm{~m}+5 \mathrm{~m}}{20 \mathrm{~s}+10 \mathrm{~s}+30 \mathrm{~s}}=\frac{12 \mathrm{~m}}{60 \mathrm{~s}}=0.2 \mathrm{~m} \mathrm{~s}^{-1}\)
\(\mathrm{A} \rightarrow \mathrm{c} \quad\) The motion of a shell fired from artillery gun is curvilinear. \(\mathrm{B} \rightarrow \mathrm{d} \quad\) A stone dropped from a tower moves vertically downwards. It is in rectilinear motion. \(\mathrm{C} \rightarrow \mathrm{b} \quad\) The wings of a ceiling fan undergo rotatory motion. \(\mathrm{D} \rightarrow \mathrm{g} \quad\) Acceleration, \(\mathrm{a}=\frac{\mathrm{v}-\mathrm{u}}{\mathrm{If}}\) body moves with uniform velocity, then \(\mathrm{v}=\mathrm{u} \Rightarrow \mathrm{a}=0 .\)
The rate of change of velocity is acceleration.
A simple pendulum that has time period of \(2 \mathrm{~s}\) is called seconds pendulum. Its length is approximately \(100 \mathrm{~cm}\) or \(1 \mathrm{~m} .\)
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