# A person travels by car from one city to another with different constant speeds betwe

• October 14th 2010, 11:37 PM
r-soy
A person travels by car from one city to another with different constant speeds betwe

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

• October 15th 2010, 12:07 AM
Educated
(a) Remember the time is given in minutes, but the velocity is given in miles per hour. You must convert one of them.

(b) Distance = velocity / time. You must work out each distance part for the different velocities.
• October 15th 2010, 01:38 AM
r-soy
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
• October 15th 2010, 02:04 AM
Educated
Quote:

Originally Posted by r-soy

(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

You have the theory correct, but you did time divided by distance instead...
Also remember to use brackets: (0.5 + 0.2 + 0.75 + 0.25)/ (80.0 + 100 + 40.0).

By the way: (0.5 + 0.2 + 0.75 + 0.25) / (80.0 + 100 + 40.0) = 0.007727272...

For the second one, I accidentally mislead you with a completely wrong formula.

Distance = velocity * time

You have to find the distance travelled for each bit of information, then add them up.
Eg. She drives for 30.0 min at 80.0 km/h

80km/h * 0.5h = 40km travelled

You must work out each bit and add the distances together to get the total distance.
• October 15th 2010, 04:35 AM
r-soy
in (0.5 + 0.2 + 0.75 + 0.25) / (80.0 + 100 + 40.0) = 0.007727272...

i didn't found like your answer when i use brackets I got

= 7.7272 X 10 ^ -12 why ?

------- part (b) ---
80km/h * 0.5h = 40km travelled
100km/h * 0.2h = 20km travelled
40km/h * 0.75h = 30km travelled

distances together to get the total distance. 40 + 20 + 30 = 90 km

plese help me .

• October 15th 2010, 04:41 AM
Soroban
Hello, r-soy!

Quote:

A person travels by car from one city to another
with different constant speeds between pairs of cities.

She drives for 30 min at 80 km/h, 12 min at 100 km/h,
and 45 min at 40 km/h and spends 15 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.

You are correct: . $\text{Average speed} \:=\:\dfrac{\text{Total distance}}{\text{Total time}}$

. . but your calculations are wrong.

Using $\text{Distance} \:=\:\text{speed} \times \text{time}$, we have:

. . $\begin{array}{ccccc}
\text{Speed} & \text{Time} & \text{Distance}\\
\text{(km/hr)} & \text{(hours)} & \text{(km)} \\ \hline
80 & 0.5 & 40 \\ 100 & 0.2 & 20 \\ 40 & 0.75 & 30 \\ 0 & 0.25 & 0 \\ \hline
\text{Total} & 1.7 & 90 \end{array}$

(b) She drove a total of 90 kilometers.

(a) Her average speed was: . $\dfrac{90}{1.7} \:\approx\:52.9\text{ km/hr}$