$(a)$ Define momentum. Write its $SI$ unit.
$(b)$ $A$ car and a truck have the same momentum. Whose velocity is more and why?
$(c)$ $A$ bullet of mass $20 \ g$ moving with a speed of $500 \ m s^{-1}$ strikes a wooden block of mass $1 \ kg$ and gets embedded in it. Find the speed with which the block moves along with the bullet.

(N/A) Momentum is defined as the product of mass and velocity of an object. Its $SI$ unit is $kg \ m s^{-1}$.
$(b)$ The formula for momentum is $p = m v$. Since the momentum $p$ is the same for both,$m_{T} v_{T} = m_{C} v_{C}$. Because the mass of the truck $m_{T}$ is greater than the mass of the car $m_{C}$,the velocity of the car $v_{C}$ must be greater than the velocity of the truck $v_{T}$ to maintain the same momentum.
$(c)$ Given: Mass of bullet $m_{1} = 20 \ g = 0.02 \ kg$,initial velocity $u_{1} = 500 \ m s^{-1}$. Mass of block $m_{2} = 1 \ kg$,initial velocity $u_{2} = 0 \ m s^{-1}$.
Applying the law of conservation of momentum: $m_{1} u_{1} + m_{2} u_{2} = (m_{1} + m_{2}) v$.
$(0.02 \times 500) + (1 \times 0) = (0.02 + 1) v$.
$10 = 1.02 \times v$.
$v = 10 / 1.02 \approx 9.8 \ m s^{-1}$.