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20) A flywheel is accelerated from rest to an angular speed of 4.5 radians/sec. If the angular displacement in doing so is 14 radians, what is the angular acceleration of the flywheel? 2 a. 0.72 rad/s? b. 2.3 rad/s? c. 5.1 rad/s d. 11 rad/s 21) Ranger Rick sees a cloud to ground lightning strike on a stormy day. 12.5 seconds later he hears the corresponding thunderous clap. Using 343 m/s as the speed of sound, how far away was the lightning strike? a. 0.274 km b. 4.29 km c. 9.80 km d. 54.6 km

Answer 1

Answer 2

SOLUTION .

Initially at rest.

Angular speed ,  ω = 4.5 radians/sec

angular displacement , 

d = ω avg. * t 

=> 14 = 4.5/2  * t

=> t = 14/2,25 = 6.2222 secs.

Angular acceleration = ω  / t = 4.5 / (14/2.25) = 0.72 radians/sec 

So, angular acceleration is 0.72 radians/sec : OPTION a.

b.

Let lightening strike is s m away.

So,

s 

= v sound * time taken to hear the sound

= 343 * 12.5 

= 4287.5 metres

= 4.29 km approx. 

So,  lightening strike point is 4.29 km away : OPTION b. (ANSWER).