Explanation:
the reason you hear a loud sound when you knock on a wall and put your ear u against it is because when you knock on said wall vibrations are bouncing off of the wall there fore making that loud banging sound.
The efficiency of a machine is the ratio between the output power and the power in input:

in our problem, the output power is 3760 W while the input power is 4000 W, so the efficiency is

So the efficiency of the machine is 94%.
Answer:
I only really know the "How do we use these EM waves in our lives?" part srry
Explanation:
EM waves are used to make sure you cellphone, radio, TV, and etc. have service/ connection.
Answer:
It takes 77 N
Explanation:
Using Newton's second law of motion, F=ma (Force equals mass times acceleration. Since the mass of the couch is 385 kg and the target acceleration is 0.2 m/s, you simply multiply mass times acceleration (ma) to get the total force, or 77 N.