Hi there!
Recall Newton's Law of Universal Gravitation:

Where:
Fg = Force of gravity (N)
G = Gravitational Constant
m1, m2 = masses of objects (kg)
r = distance between objects (m)
Plug in the given values stated in the problem:

Answer:
After the firing occurs, both the cannon and cannonball have the same momentum (big mass, small velocity vs. small mass, big velocity). But since the momentum for each is moving in the opposite direction, the momentums cancel out, causing the cannon-cannonball system's momentum to equal zero.
Explanation:
The law of conservation of ENERGY
20 because if you go to 45 then scroll up it is at 20 degrees celcius
Answer:
400 N
Explanation:
Due to action-Reaction, the wall pushes back on the person with a force of equal magnitude and in opposite direction to the force exerted by the person.
So the magnitude must be also 400 N.