Question

If the system is released from rest, find the contact force between blocks of masses $3kg$ and $1kg$. Assume that all surfaces are frictionless, pulley and threads are massless. (Take $g=10m/s^2$)

• A. $2N$
• B. $3N$
• C. $4N$
• D. $5N$

Answer 1

The correct option is D $5N$

The acceleration of the blocks is given by:
$a=\frac{f_{pulling}}{\sum M}=\frac{80}{16}=5m/s^2$

Now, if you consider block of mass $1kg$,
Here, $\text{N}$ is the contact force between blocks of masses $3kg$ and $1kg$.
Only one force is responsible for causing acceleration of block of mass $1kg$, which is $\text{N}$. So,
$\text{N}=ma$
$\text{N}=1\times 5=5N$