Answer:
An atmosphere is the layers of gases surrounding a planet or other celestial body. Earth's atmosphere is composed of about 78% nitrogen, 21% oxygen, and one percent other gases
Answer:
Systematic errors.
Explanation:
The density of the aluminium was calculated by a human and this is not natural but can be due to errors in the calibration of the scale for measuring the weight or taking readings from the measuring cylinder.
Random errors are natural errors. Random errors in experimental measurements are caused by unknown and unpredictable changes in the experiment. Systematic errors are due to imprecision or problems with instruments.
Explanation:
Efficiency is defined as the ratio between the useful output over the total amount consumed. 
The fan does 500W of useful work while wasting 300 W. The total power consumption is 800 W (500 + 300).
That's called "refraction".
Answer:
the answer is
Explanation:For equilibrium
Weight = Tension
mg=T
∴T=4×3.1π=12.4πN (as can be inferred from the question)
Y=
△l/l
T/A
=
1000
0.031
/20
12.4π/π(
1000
2
)
2
=
4×0.031
12.4×20×1000×(1000)
2
=2×10
12
N/m
2
