Question

Two blocks of masses $2\text{kg}$ and $5\text{kg}$ are at rest on ground. The masses are connected by a string passing over a frictionless light pulley which is under the influence of a constant upward force $F=50\text{N}$. The acceleration of $5\text{kg}$ and $2\text{kg}$ masses are

• A. $0,2.5{\text{ms}}^{-2}$
• B. $0,0$
• C. $2.5{\text{ms}}^{-2},2.5{\text{ms}}^{-2}$
• D. $1{\text{ms}}^{-2},2.5{\text{ms}}^{-2}$

Answer 1

The correct option is A $0,2.5{\text{ms}}^{-2}$

The masses will be lifted if the tension of the string is greater than the gravitational pull on masses.
Weight of $5\text{kg}$ mass $=5\times 10=50\text{N}$ and $2\text{kg}$ mass $=2\times 10=20\text{N}$
From free body diagram
$50-2T=0$
or $T=25\text{N}$
So $5\text{kg}$ weight can not be lifted $(\because \text{acceleration}=0)$
but $2\text{kg}$ weight will be lifted.
$\therefore 25-20=2a$ or $a=\frac{5}{2}=2.5{\text{ms}}^{-2}$