Question

A particle was at rest from 9am.it moved at a uniform speed 10km/hr from 9.30am to 10am.find the average speed between a 9am to 10am b 9.15 to 10 am.

Answer 1

The speed of particle started after 9:30 am, and it was at rest from 9 am, so
speed of particle from 9 am to 9:30 am = 0

And speed of particle from 9:30 am to 10 am = 10 km/hr
Average speed = Total distance / total time
(a) for 9 am to 10 am
total distance = speed x time
= 10 x 1/2  (as particle was at rest till 9:30 am)
total dist = 5 km
total time = 1 hr
avg speed = 5/1 = 5km/hr
(b) for 9:15 am to 10 am
total distance = speed x time
= 10 x 1/2  (as particle was at rest till 9:30 am)
total dist = 5 km
total time = 3/4 hr
avg speed = 5/(3/4) = 20/3 km/hr = 6.66 km/hr

Answer 2

Given conditions ⇒

Speed of the Particles = 10 km/hr. 

As per as the Question, we need to find the Average speed of the Particles between 9 am to 10 am. 
∴ Total time = 1 hrs. 
Speed of the Particles between 9 to 9:30 am = 0 

∴ Distance covered by the Particles in 30 minutes = 10 × 0.5
 = 5 km.  

Average speed = Total Distance/Total time. 
∴  = 5 × 1 
    = 5 km/hr. 

Hence, the average speed of the Particles is 5 km/hr. 

___________________________

(b). Distance covered in 30 minutes = 5 km. 
Total time taken by it = 45 minutes. 
 = 45/60 hrs. 
 = 3/4 hrs. 

∴ Average Speed = 5/(3/4)
    = 20/3 
    = 6.67 km/hr. 


Hence, the average speed of the Particles is 6.67 km/hr. 



Hope it helps. :-)