<u>Answer:</u> The momentum of the car is 16000 kg.m/s
<u>Explanation:</u>
Momentum is defined as the force that keeps the object moving. It is also defined as the product of mass and velocity of an object.
Mathematically,

where,
p = momentum of the car = ?
m = mass of the car = 800 kg
v = velocity of the car = 20 m/s
Putting values in above equation, we get:

Hence, the momentum of the car is 16000 kg.m/s
Answer:
(a) 
(b) 
Explanation:
We can derive the initial speed of the rock from the equation of the speed in function of the time:

Using the given values for the speed at time t=1.7s, we get:

In words, the speed of the rock at launch is 34m/s (a).
Next, we use this to calculate the speed at t=4.9s:

This means that the speed of the rock at 4.9s after the launch is 14m/s (b), and the negative sign means that it is moving downwards.