MHT CET · Physics · Gravitation
A body is projected vertically, upwards from earth's surface with velocity \(2 V_e\) where \(V_e\) is the escape velocity from earth's surface. The velocity when body escapes the gravitational pull is
- A \(\sqrt{3} V_e\)
- B \(V_e\)
- C \(\sqrt{2} V_e\)
- D \(\frac{V_e}{\sqrt{3}}\)
Answer & Solution
Correct Answer
(A) \(\sqrt{3} V_e\)
Step-by-step Solution
Detailed explanation
To escape the gravitational pull \(F=\frac{G M m}{r^2}\), the body needs to be at
\(\begin{aligned} & r \rightarrow \infty \\ & \Rightarrow-\left(\frac{G M m}{R}\right)+\frac{1}{2} m\left(V_e\right)^2=0 \\ & \Rightarrow V_e=\sqrt{\frac{2 G M}{R}}\end{aligned}\)
Therefore, if the particle is projected with \(2 V_e\) from the surface of earth, then at \(r \rightarrow \propto\) the velocity of the projectile is \(V\). Considering energy conservation:
\(\begin{aligned} & \Rightarrow-\left(\frac{G M m}{R}\right)+\frac{1}{2} m\left(2 V_e\right)^2=0+\frac{1}{2} m V^2 \\ & \Rightarrow-\frac{G M}{\mathrm{R}}+\frac{1}{2}\left(2 V_e\right)^2=\frac{1}{2} V^2 \\ & \Rightarrow-\frac{G M}{\mathrm{R}}+\frac{4 G M}{R}=\frac{1}{2} V^2 \\ & \therefore V=\sqrt{\frac{6 G M}{R}}=\sqrt{3}\left(\sqrt{\frac{2 G M}{R}}\right)=\sqrt{3} V_e\end{aligned}\)
\(\begin{aligned} & r \rightarrow \infty \\ & \Rightarrow-\left(\frac{G M m}{R}\right)+\frac{1}{2} m\left(V_e\right)^2=0 \\ & \Rightarrow V_e=\sqrt{\frac{2 G M}{R}}\end{aligned}\)
Therefore, if the particle is projected with \(2 V_e\) from the surface of earth, then at \(r \rightarrow \propto\) the velocity of the projectile is \(V\). Considering energy conservation:
\(\begin{aligned} & \Rightarrow-\left(\frac{G M m}{R}\right)+\frac{1}{2} m\left(2 V_e\right)^2=0+\frac{1}{2} m V^2 \\ & \Rightarrow-\frac{G M}{\mathrm{R}}+\frac{1}{2}\left(2 V_e\right)^2=\frac{1}{2} V^2 \\ & \Rightarrow-\frac{G M}{\mathrm{R}}+\frac{4 G M}{R}=\frac{1}{2} V^2 \\ & \therefore V=\sqrt{\frac{6 G M}{R}}=\sqrt{3}\left(\sqrt{\frac{2 G M}{R}}\right)=\sqrt{3} V_e\end{aligned}\)
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