Solution :
We assume that there is a ring having a charge +Q and radius r. Electric field due to the ring at a point P on the axis is given by :




If we put an electron on point P, then force on point e is :

![F= \frac{-eKQx}{(r^2+x^2)^{3/2}}= \frac{-eKQx}{r^3[1+\frac{x^2}{r^2}]^{3/2}}](https://tex.z-dn.net/?f=F%3D%20%5Cfrac%7B-eKQx%7D%7B%28r%5E2%2Bx%5E2%29%5E%7B3%2F2%7D%7D%3D%20%5Cfrac%7B-eKQx%7D%7Br%5E3%5B1%2B%5Cfrac%7Bx%5E2%7D%7Br%5E2%7D%5D%5E%7B3%2F2%7D%7D)
If r >> x , then 
Then, 


Compare, a = -ω²x
We get,




Answer:
23.086 mile/h
Explanation:
Given,
Distance Tyson Gay run = 100 m
time of run, t = 9.69 s
average speed of the in mph = ?
Speed of the Gay = 

v = 10.32 m/s
1 m = 3.281 ft
10.32 m = 33.86 ft
1 mile = 5280 ft
1 ft = 1.8939 x 10⁻⁴ mile
33.86 ft/s = 6.413 x 10⁻³ miles/s
Speed of Tyson in mile/hr = 6.413 x 10⁻³ x 3600
= 23.086 mile/h
Hence, speed of Tyson Gay's in mile/ hr is equal to 23.086 mph.
First speed = 20km/h
Time = 3 hours
Distance = 3×20
<h3> = <u>60 km</u></h3>
Second speed = 30km/h
Time = 4 hours
Distance = 4×30
<h3> = <u>120 km</u></h3>
Total distance = 60+120 = <u>180km</u>
Total time = 3+4 =<u> 7 hours</u>
Average speed = 180/7
<h3> = <u>25.71</u><u> </u><u>km</u><u>/</u><u>h</u></h3>
Hope this will help...