Answer:
-48 N
Explanation:
mass of door (m) = 4 kg
acceleration of the door = 12 m/s^{2}
force exerted by the person = 48 N
From Newton's third law of motion, action and reaction are equal but opposite. Therefore the force exerted on the door by the person which is 48 N will be the same as the force exerted on the person by the door but opposite in its direction, and this would be - 48 N

Actually Welcome to the concept of Efficiency.
Here we can see that, the Input work is given as 2.2 x 10^7 J and the efficiency is given as 22%
The efficiency is => 22% => 22/100.
so we get as,
E = W(output) /W(input)
hence, W(output) = E x W(input)
so we get as,
W(output) = (22/100) x 2.2 x 10^7
=> W(output) = 0.22 x 2.2 x 10^7 => 0.484 x 10^7
hence, W(output) = 4.84 x 10^6 J
The useful work done on the mass is 4.84 x 10^6 J
Change in state(from liquid to solid) and change in colour I believe.
Answer:
C. Pressure gradient equals gas flow over resistance.
Explanation:
As we know that pressure gradient is the driving force for the gas to flow from one point to other point
And we know that the flow rate is directly proportional to the driving force and it inversely depends on the resistance to flow
so we can say
Flow Rate = 
Flow Rate = 
so we can say that correct statements are as below
A. Gas flow equals pressure gradient over resistance.
B. Resistance equals pressure gradient over gas flow.
D. The amount of gas flowing in and out of the alveoli is directly proportional to the difference in pressure or pressure gradient between the external atmosphere and the alveoli.