Question:

A running man has half the kinetic energy of that of a boy of half of his mass. The man speeds up by $ 1\,m/s $ , so as the have same kinetic energy as that of the boy. The original speed of the man is:

Updated On: Jun 20, 2022
  • $ \left( \sqrt{2}-1 \right)m/s $
  • $ \sqrt{2}\,m/s $
  • $ \frac{1}{\sqrt{2}-1}\,m/s $
  • $ \frac{1}{\sqrt{2}}\,m/s $
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The Correct Option is C

Solution and Explanation

The kinetic energy of a moving body is equal to half the product of the mass $(m)$ of the body and the square of its speed $\left(v^{2}\right)$.
Kinetic energy $ =\frac{1}{2} \times \text { mass } \times(\text { speed })^{2} $
i.e., $=\frac{1}{2} m v^{2}$
Let mass of man is $M\, kg$ and speed is $v$ and speed of boy is $v_{1}$, then
$\frac{1}{2} M v^{2}=\frac{1}{2}\left(\frac{1}{2} \frac{M}{2} v_{1}^{2}\right)\,\,\,...(1)$
When man speeds up by $1 m / s$, then
$v =v+1 $
$\therefore \frac{1}{2} M(v+1)^{2} =\frac{1}{2}\left(\frac{M}{2}\right) \cdot v_{1}^{2}\,\,\,...(2)$
Dividing E (1) by (2), we get
$ \frac{v^{2}}{(v+1)^{2}} =\frac{1}{2} $
$\Rightarrow \frac{v}{v+1} =\frac{1}{\sqrt{2}} $
$\Rightarrow \sqrt{2} v =v+1 $
$\Rightarrow v =\frac{1}{\sqrt{2}-1} \,m / s$
Note : In the formula for kinetic energy speed occurs in the second power and so has a larger effect compared to mass.
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Concepts Used:

Work, Energy and Power

Work:

  • Work is correlated to force and the displacement over which it acts. When an object is replaced parallel to the force's line of action, it is thought to be doing work. It is a force-driven action that includes movement in the force's direction.
  • The work done by the force is described to be the product of the elements of the force in the direction of the displacement and the magnitude of this displacement.

Energy:

  • A body's energy is its potential to do tasks. Anything that has the capability to work is said to have energy. The unit of energy is the same as the unit of work, i.e., the Joule.
  • There are two types of mechanical energy such as; Kinetic and potential energy.

Read More: Work and Energy

Power:

  • Power is the rate at which energy is transferred, conveyed, or converted or the rate of doing work. Technologically, it is the amount of work done per unit of time. The SI unit of power is Watt (W) which is joules per second (J/s). Sometimes the power of motor vehicles and other machines is demonstrated in terms of Horsepower (hp), which is roughly equal to 745.7 watts.
  • Power is a scalar quantity, which gives us a quantity or amount of energy consumed per unit of time but with no manifestation of direction.