Homework Help: Physics speed homework

1. Jun 2, 2009

balling12

1. The problem statement, all variables and given/known data
A person takes a trip, driving with a constant speed of 91.0 km/h except for a 22.0 min rest stop. If the person's average speed is 62.4 km/h, how much time is spent on the trip and how far does the person travel?

2. Relevant equations

3. The attempt at a solution
not sure how to begin

2. Jun 2, 2009

cepheid

Staff Emeritus
Re: stumped

You haven't put anything under the relevant equations. The problem asks you to deduce distance travelled and time spent based on an average speed. What is the definition of average speed?

3. Jun 2, 2009

dx

Re: stumped

Let T be the total time that he drives (not including the rest stop), and D the total distance that he travels. You know that D/T = 91 km/h. That's one equation.

What is the average speed in terms of D and T? This will be your second equation.

You can solve these two equations for D.

4. Jun 2, 2009

balling12

Re: stumped

average speed= total distance/total time

5. Jun 2, 2009

cepheid

Staff Emeritus
Re: stumped

Right, and how does the total time relate to the time that he drives (which is T using dx's notation)?

6. Jun 2, 2009

balling12

Re: stumped

T-22 minutes?

7. Jun 2, 2009

cepheid

Staff Emeritus
Re: stumped

Why subtraction? Are you saying that the total time for the trip is LESS than the time spent driving?

If I spend T minutes driving *and* 22 minutes sitting at a rest stop, what is the total time I spend on the trip?

8. Jun 2, 2009

balling12

Re: stumped

ok so it would be the total time(T)+22. sorry i got that confused. But i am still unsure were to go from there?

9. Jun 2, 2009

cepheid

Staff Emeritus
Re: stumped

Remember that you have two equations and two unknowns. The unknowns are:

distance travelled (D)

time spent driving (T)

The two equations are:

driving speed = 91 km/h = distance driven/time spent driving = D/T

average speed (by *definition*) = 64 km/h = distance travelled / TOTAL time = D/(T+22)

Two equations, two unknowns. This means that you can get a unique solution for both D and T.

10. Jun 2, 2009

cepheid

Staff Emeritus
Re: stumped

Here are the equations more cleanly:

91 = D/T

64 = D/(T+22)

11. Jun 2, 2009

dx

Re: stumped

It's better to write 22 minutes as 22/60 hours to keep the units consistent. Then you will get the answers for D and T in kilometers and hours respectively.