Register to reply 
A person travels by car from one city to another with different constant speeds 
Share this thread: 
#1
Oct1510, 02:38 AM

P: 159

A person travels by car from one city to another with different constant speeds between pairs of cities. She drives for 30.0 min at 80.0 km/h, 12.0 min at 100 km/h, and 45.0 min at 40.0 km/h and spends 15.0 min eating lunch and buying gas. (a) Determine the average speed for the trip. (b) Determine the distance between the initial and final cities along the route.
I try to solve (a) Averge speed = total distance / total time = 30 + 12 + 45 + 15 / 80.0 + 100 + 40.0 = 0.1673 km/h (b) The distance = 80 + 100 + 40 = 220 km please I wan t to correct my answer . 


#2
Oct1510, 02:42 AM

Admin
P: 23,406

This is wrong. How far do you get driving 30 minutes at 80 km/h?



#3
Oct1510, 04:22 AM

P: 159

which one is wrong and I fo same the
Averge speed = total distance / total time ?? 


#4
Oct1510, 04:38 AM

P: 159

A person travels by car from one city to another with different constant speeds
I try to solve agine
(a) Averge speed = total distance / total time now i try to convert all the time to hour by using 1 h = 60 30 /60 = 0.5 and 12/60 = 0.2 and 45/60 = 0.75 and 15/60 = 0.25 0.5 + 0.2 + 0.75 + 0.25/ 80.0 + 100 + 40.0 = 1.4511 km/h (b) The distance = Distance = velocity / time = 1 ) 40  12 / 1.95 = 14.35 m/s 


#5
Oct1510, 07:24 AM

P: 159

where are you ?



#6
Oct1510, 07:29 AM

P: 423

you know
Averge speed = total "distance" / total time are you sure that's what you have done in your solution? 


#7
Oct1510, 10:00 AM

Admin
P: 23,406

Summing speeds is not a way of calculating distance. The only thing you approached correctly so far was calculation of total time. No idea why you try to calculate average speed dividing time by the distance, instead of distance by the time. To calculate total distance you have to separately calculate distance covered with each speed, then sum them. For example, if you drive for half an hour at 40 km/h and then for 15 minutes at 100 km/h, you cover 0.5h*40km/h=20km in the first leg and 0.25h*100km/h=25km in the second leg, total of 45 km. 


Register to reply 
Related Discussions  
350 Hz sound travels through air and metal pipe at different speeds.  Introductory Physics Homework  2  
Speeds of a Person  Introductory Physics Homework  2  
A plane flies 457 km east from city A to city B  Introductory Physics Homework  3  
City Life OWNZ Sim City  General Discussion  0  
Quantizing speeds with the cosmological constant  Beyond the Standard Model  16 