- Chapter 1 – Measurements and Experimentation
- Chapter 2 Motion In One Dimension
- Chapter 3 Laws Of Motion
- Chapter 4 Pressure In Fluids And Atmospheric Pressure
- Chapter 5 Upthrust In Fluids Archimedes Principle And Floatation
- Chapter 6 Heat And Energy
- Chapter 7 Reflection Of Light
- Chapter 8 Propogation Of Sound Waves
- Chapter 9 Current Electricity
- Chapter 10 Magnetism
Selina Solutions Class 9 Physics Solutions for Exercise 3E in Chapter 3 - Chapter 3 Laws Of Motion
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(a) How long will a stone take a fall to the ground from the top of a building 80m high and (b) what will be the velocity of the stone on reaching the ground? (Take g=10m/s^2)
Given: height=80m, g=10m/s^2
(a)From the equation of motion;
\begin{array}{l} \mathrm{H}=\mathrm{ut}+1 / 2 \mathrm{gt}^{2} \\ \Rightarrow 80=0(\mathrm{t})+1 / 2(10) \mathrm{t}^{2} \\\Rightarrow 160=10 \mathrm{t}^{2} \\ \Rightarrow \mathrm{t}=4 \mathrm{s} \end{array}
(b) To find the velocity of the stone, assume ‘v’ to be the velocity
From the equation of motion;
\begin{array}{l} \mathbf{v}^{2}-\mathbf{u}^{2}=2 \mathrm{gh} \\ \mathbf{v}^{2}-0=2(10)(80) \\ \mathbf{v}^{2}=1600 \\ \mathbf{v}=40 \mathrm{m} / \mathrm{s} \end{array}
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