A man (mass =50 kg ) and his… - JEE Main 2019 Physics PYQ

JEE Main 2019 — Physics Mechanics

2019mcqeasy

A man (mass $=50 kg$ ) and his son (mass $=20 kg$ ) are standing on a frictionless surface facing each other. The man pushes his son so that he starts moving at a speed of $0.70 m {s}^{-1}$ with respect to the man. The speed of the man with respect to the surface is:

Official previous-year question

Held on 12 Apr 2019 · Verified 6 Jul 2026.

Options

A
$0.20 m {s}^{-1}$
B
$0.14 m {s}^{-1}$
C
$0.47 m {s}^{-1}$
D
$0.28 m {s}^{-1}$

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Solution

By conservation of linear momentum,

${\vec{P}}_{i}={\vec{P}}_{f}$

Where ${\vec{P}}_{i}$ and ${\vec{P}}_{f}$ are initial and final momentum of system (boy + man)

Now, $0={m}_{b}{\vec{v}}_{b}+{m}_{m}{\vec{v}}_{m}$ ......(i)

${v}_{b}=$ velocity of boy w.r.t. ground

${v}_{m}=$ velocity of man w.r.t. ground

${\vec{v}}_{\mathrm{bm}}=$ velocity of boy w.r.t. man

So, ${\vec{v}}_{\mathrm{bm}}={\vec{v}}_{b}-{\vec{v}}_{m}$

or ${\vec{v}}_{b}={\vec{v}}_{\mathrm{bm}}+{\vec{v}}_{m}$

or ${\vec{v}}_{b}=0.7+{\vec{v}}_{m}$ ........(ii)

From equation (i) and (ii),

$0=20(0.7+{\vec{v}}_{m})+50{\vec{v}}_{m}$

${\vec{v}}_{m}=-\frac{14}{70}=-0.20m{s}^{-1}$

Negative sign shows velocity of man is opposite to boy.

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Solution

By conservation of linear momentum,

${\vec{P}}_{i}={\vec{P}}_{f}$

Where ${\vec{P}}_{i}$ and ${\vec{P}}_{f}$ are initial and final momentum of system (boy + man)

Now, $0={m}_{b}{\vec{v}}_{b}+{m}_{m}{\vec{v}}_{m}$ ......(i)

${v}_{b}=$ velocity of boy w.r.t. ground

${v}_{m}=$ velocity of man w.r.t. ground

${\vec{v}}_{\mathrm{bm}}=$ velocity of boy w.r.t. man

So, ${\vec{v}}_{\mathrm{bm}}={\vec{v}}_{b}-{\vec{v}}_{m}$

or ${\vec{v}}_{b}={\vec{v}}_{\mathrm{bm}}+{\vec{v}}_{m}$

or ${\vec{v}}_{b}=0.7+{\vec{v}}_{m}$ ........(ii)

From equation (i) and (ii),

$0=20(0.7+{\vec{v}}_{m})+50{\vec{v}}_{m}$

${\vec{v}}_{m}=-\frac{14}{70}=-0.20m{s}^{-1}$

Negative sign shows velocity of man is opposite to boy.

Options

Did you get this right?

Solution

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Options

Did you get this right?

Solution

Your note

JEE Main Mechanics in other years

More JEE Main Mechanics questions

Other Physics topics for JEE Main

Electromagnetism

Modern Physics

Optics

Thermodynamics

Waves & Oscillations

Keep practicing Mechanics