What is the Average Speed of a Car Traveling Up and Down Hills?

[mossfan563]

Homework Statement

A car travels up a hill at 31 km/h and then down a hill at 59 km/h.
Find the avg speed.

Homework Equations

The Attempt at a Solution

I tried adding the speeds and then dividing by 2 but I guess that's definitely not how to approach this problem.

How do I go about starting this problem?

 

[Defennder]

?? That's all you're given? I thought average speed was total dist/total time.

 

[mossfan563]

?? That's all you're given? I thought average speed was total dist/total time.

Yeah all I'm given is that those speeds are constant and they want the avg speed for the round trip.

Maybe "round trip" gives something away.

 

[mossfan563]

Anybody?

 

[Ygggdrasil]

Assume the car travels x miles uphill and x miles downhill. Find the total time and divide 2x by the total time.

 

[Doc Al]

Homework Statement

A car travels up a hill at 31 km/h and then down a hill at 59 km/h.
Find the avg speed.

I presume that the distance up the hill equals the distance down the hill. (Without some such assumption, there would be insufficient information.)

What's the most basic definition of average speed? Use it.

 

[mossfan563]

Assume the car travels x miles uphill and x miles downhill. Find the total time and divide 2x by the total time.

Ok I get a really small number. Is that legit?

I assumed the car traveled 10 km up and down and i divided by 20 (2x).
I got .024603...
Is that correct? Or am I doing something wrong?

 

[mossfan563]

I presume that the distance up the hill equals the distance down the hill. (Without some such assumption, there would be insufficient information.)

What's the most basic definition of average speed? Use it.

The basic definition is total distance/total time. I have no instance of time and no distance either.

 

[Doc Al]

The basic definition is total distance/total time.

Good.

I have no instance of time and no distance either.

Use Ygggdrasil's hint: Call the distance x (or D, if you like). In terms of the distance, find the time for going up and the time for going down.

 

[Ygggdrasil]

Ok I get a really small number. Is that legit?

I assumed the car traveled 10 km up and down and i divided by 20 (2x).
I got .024603...
Is that correct? Or am I doing something wrong?

Check the units on your calculation. You'll see where your calculation went wrong.

 

[mossfan563]

I see where i went wrong. I divided the other way around.
Does 40.666666666... sound right now?

 

[Ygggdrasil]

Yup. Of course, this assumes that the distance up the hill is equal to the distance down the hill, which may or may not be a correct assumption.

 

[mossfan563]

It is correct. Thanks for the help!