Suppose you are running fast in a field when you suddenly find a snake in front of you. You stop quickly. Which force is responsible for your deceleration?

A plank of mass $m_1=8 \mathrm{kg}$ with a bar of mass $m_2=2 \mathrm{kg}$ placed on its rough surface, lie on a smooth floor of elevator ascending with an acceleration $g/4$. The coefficient of friction is $\mu =1/5$ between $m_1$ and $m_2$. A horizontal force $F=30 \mathrm{N}$ is applied to the plank. Then the acceleration of bar and the plank in the reference frame of elevator are:

A steel ball of mass $0.5 \mathrm{kg}$ is dropped from a height of $4 \mathrm{m}$ on to a horizontal heavy steel slab. The ball strike the slab and rebounds to its original height. $\left(\mathrm{Take} g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$

(a) Calculate the impulse delivered to the ball during impact.

(b) If the ball is in contact with the slab for $0.002 \mathrm{s},$ find the average reaction force on the ball during impact.

Draw F.B.D. of the sphere of mass $M$ placed between a vertical wall and ground as shown in the figure (all surfaces are smooth).