Question

A proton moving with a velocity of $1.25\times {10}^5$m / s collides with a stationary helium atom. The velocity of proton after collision is

• A. $0.75\times {10}^{5}m{s}^{-1}$
• B. $7.5\times {10}^{5}m{s}^{-1}$
• C. $-7.5\times {10}^{5}m{s}^{-1}$
• D. $0m{s}^{-1}$

Answer 1

Answer: (C)

$-7.5\times {10}^{5}m{s}^{-1}$

Using momentum conservation,
$mv=4m{v}_2+m{v}_1$

$e=\frac{v_2-v_1}{v}=1$

$v_2-v_1=v$
$4v_2+v_1=v$

$v_2=\frac{2}{5}v$

$v_1=\frac{-3}{5}v$

$=\frac{-3}{5}\times 1.25\times {10}^5$

$=-7.5\times {10}^5$ m/s