Why Did My Calculation of Driving Speed Result in an Error?

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Homework Help Overview

The problem involves calculating the average speed required for a car after jogging a certain distance, with the goal of achieving a specific overall average speed for the entire journey. The context is within the subject area of kinematics.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of time taken for jogging and the subsequent average speed needed for driving. There are questions about the accuracy of the calculations and the use of units, particularly the conversion between kilometers and miles.

Discussion Status

Some participants are attempting to clarify the calculations and identify potential errors, particularly in the value of x and the use of units. There is a mix of confidence in the calculations and requests for further clarification, indicating an ongoing exploration of the problem.

Contextual Notes

There is mention of a test that is modeled after the homework, which may add urgency to the discussion. Participants are also questioning the appropriateness of using miles in the calculations.

Manodesi524
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Homework Statement



You jog at 9.8 km/ h for 8.0 km, and then you jump in the car and drive an additional 16km. With what average speed must you drive the car if your avg speed for the entire 24 km is to be 22km/ h

Homework Equations


Average speed = distance/ time


The Attempt at a Solution



I found the time it took for my jog, which was approx. .84 miles.
Then I made an equation using some facts :

22= 24/ .84+ x

X is about .25 hours

So now that I know this I plug it into my equation that will solve the problem: 16/.25

My final answer is about 64 km/ hr.

My teacher says its 50. Where did I go wrong?
 
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You made an error calculating x. If 22 = 24 / (.84 + x), we know that the denominator for that equation must be equal to 24 / 22. Calculate that, then subtract .84 to get the time in the car. Then go from there.
 
Mark, i am sure x is .25. I've done the calculations several times. I would like an answer soon because today's test is modeled after the homework
 
Manodesi524 said:
I found the time it took for my jog, which was approx. .84 miles.
0.8163 hrs

My final answer is about 64 km/ hr.

My teacher says its 50. Where did I go wrong?
I say it's about midway between those values. Your method is correct, just need to retain more sig figs.
 
Manodesi524 said:

Homework Statement



You jog at 9.8 km/ h for 8.0 km, and then you jump in the car and drive an additional 16km. With what average speed must you drive the car if your avg speed for the entire 24 km is to be 22km/ h

Homework Equations


Average speed = distance/ time


The Attempt at a Solution



I found the time it took for my jog, which was approx. .84 miles.
Then I made an equation using some facts :

22= 24/ .84+ x

X is about .25 hours

So now that I know this I plug it into my equation that will solve the problem: 16/.25

My final answer is about 64 km/ hr.

My teacher says its 50. Where did I go wrong?
Why would you use miles to solve any part of this problem?
 

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