Answer:

Explanation:
Mass of block=10 kg
Applied horizontal force =F=20 N
Friction force=f=10 N
We have to find the acceleration of block.
Net force=Applied horizontal force-friction force

Where F= Horizontal force
f=Friction force
m=Mass of object
a=Acceleration of object


Hence, the acceleration of the block=
Answer:
They can't hear an echo in small room because in it the sound can't be reflected back. For an echo of a sound to be heard,the minimum distance between the source of sound and the walls of the room should be 17.2 m.
hopw it helps
Answer:
The stress is calculated as 
Solution:
As per the question:
Length of the wire, l = 75.2 cm = 0.752 m
Diameter of the circular cross-section, d = 0.560 mm = 
Mass of the weight attached, m = 25.2 kg
Elongation in the wire, 
Now,
The stress in the wire is given by:
(1)
Now,
Force is due to the weight of the attached weight:
F = mg = 
Cross sectional Area, A = 
Using these values in eqn (1):
A) Expanding. We know this because it has a similar effect with sound. When a car goes by the pitch gets deeper and deeper. It's because you're receiving less waves. Same thing for light but instead of a pitch it's light, and the farther spread the waves - the redder, the closer and more contracted - the bluer