Answer:
Part a)

Part b)

Part c)

Explanation:
Part a)
As we know that the friction force on two boxes is given as



Now we know by Newton's II law

so we have




Part b)
For block B we know that net force on it will push it forward with same acceleration so we have




Part c)
If Alex push from other side then also the acceleration will be same
So for box B we can say that Net force is given as




As the first astronaut throws the ball, lets assume it goes with v velocity and the mass of the ball be m
the momentum comes out be mv, thus to conserve that momentum the astronaut will move opposite to the direction of the ball's motion with the velocity mv/M (where M is the mass of the astronaut).
Yes it may or may not be depending on the angle of the ramp. But if nothing is specified then it must be related to acceleration as friction is neglected.
Answer:

Explanation:
Speed of van, v = 28 m/s
Radius of unbanked curve, r = 150 m
Let
is the angle with the vertical. In case of banking of road,
.............(1)
And
..........(2)
From equation (1) and (2) :



So, the string makes an angle of 28.07 degrees with the vertical. Hence, this is the required solution.