A force of $50 \mathrm{N}$ acts on a body initially at rest. Find the momentum of the body at $3$ second and $6$ seconds after the force starts acting.

A bullet of mass $5 \mathrm{g}$ travelling with a velocity of $18 \mathrm{km}/\mathrm{h}$ penetrates a wooden block and comes to rest in $0.01 \mathrm{s}$. Find

$\left(i\right)$ the distance through which it penetrates in the wooden block;

$\left(ii\right)$ the value of retarding force of the block.

Two blocks of mass $m_1=1 \mathrm{kg}$ and $m_2=2 \text{kg}$ are placed in contact on a frictionless horizontal surface. A force of $10 \text{N}$ is acting on $m_1$. What is the acceleration of $m_1$ and $m_2$. What is the value of action and reaction between the blocks?

A constant retarding force of $100 \mathrm{N}$ is applied to a body of mass $60 \mathrm{kg}$ moving initially with a speed of $18 \mathrm{km}/\mathrm{h}$. How long does the body take to come to halt? 

Explain balanced and unbalanced forces giving suitable examples.

Describe Galileo's experiments with marbles on the inclined planes and write its conclusion.

State principle of conservation of linear momentum. Illustrate by an example.