The answer is reflection.
The drawing is simple but illustrates the concept beautifully.
Albert Einstein came up with the theory of general relativity to explain the law of gravity, whilst Newton's three laws of gravity is universal. To understand this further, it's best to understand it in scientific terms.
The weird thing about science is that words that are used in a colloquial sense may have a completely opposite definition in scientific terms.
A law in science is a constant and invariable statement that is universal. Wherever you may be in the universe, Newton's three laws of gravity will always be applied.
The word "theory" doesn't imply conjecture or an idea someone made up after a night of drinking. In science, a theory is the highest level of certainty behind mathematical proof -- which isn't even a part of science, obviously. A theory has to be substantiated by all available evidence and contradicted by none. All theories also have to have to be falsifiable. For this reason, theories can never be proven. Einstein's theory of general relativity has great predictive power, but in some cases, the predictions aren't always constant. Theories are often revised to fit new available evidence.
Part a)
per day electricity power consumed when 100 W bulb is used for 8 hours

for one year consumption

now the cost will be given

now when other energy efficient light is used

for one year consumption

now the cost will be given

Answer:
t=1.623 sec
Explanation:
The distance traveled before the echo is had is:

Given the speed of sound as v=345m/s, we use the speed equation to solve for t:

Hence, it takes 1.623 seconds to hear the echo.
Answer:Gravity
Explanation:Gravity is the force that pulls everything down instead of up because if we didn’t have gravity we would be floating upwards