Answer:
20 mins
Explanation:
To go 90 km it takes =60 mins(1 hr)
" " 1 km it takes =60/90 mins
" " 30 km it takes =60/90 multiplied by 30
 
 = 20 mins
 
        
             
        
        
        
Answer:

Explanation:
When teacher falls into the cliff then she shout for Help at the same time
so here we know that sound will go down and reflect back up
so here in 3 s distance traveled by the sound



now in the same time the distance that teacher will fall down is given as



now total distance traveled by teacher and sound in 3 s


this total distance must be equal to twice the height of the cliff


 
        
                    
             
        
        
        
Answer:![F_{net}=\frac{kq^2}{(L)^2}\left [ \frac{1}{2}+\sqrt{2}\right ]](https://tex.z-dn.net/?f=F_%7Bnet%7D%3D%5Cfrac%7Bkq%5E2%7D%7B%28L%29%5E2%7D%5Cleft%20%5B%20%5Cfrac%7B1%7D%7B2%7D%2B%5Csqrt%7B2%7D%5Cright%20%5D)
Explanation:
Given
Three charges of magnitude q is placed at three corners and fourth charge is placed at last corner with -q charge 
Force due to the charge placed at diagonally opposite end on -q charge

where   Distance between the two charges
Distance between the two charges

negative sign indicates that it is an attraction force
Now remaining two charges will apply the same amount of force as they are equally spaced from -q charge

The magnitude of force by both the  charge is same but at an angle of 
thus combination of two forces at 2 and 3 will be 

Now it will add with force due to 1 charge 
Thus net force will be 
![F_{net}=\frac{kq^2}{(L)^2}\left [ \frac{1}{2}+\sqrt{2}\right ]](https://tex.z-dn.net/?f=F_%7Bnet%7D%3D%5Cfrac%7Bkq%5E2%7D%7B%28L%29%5E2%7D%5Cleft%20%5B%20%5Cfrac%7B1%7D%7B2%7D%2B%5Csqrt%7B2%7D%5Cright%20%5D)
 
        
             
        
        
        
The speed of the car is exactly 150/7200 km/sec, or 125/6 meters/sec.  
In more familiar units, that speed is equivalent to ...
-- (20 and 5/6) meters/sec
-- 75 km/hour