Calculating Average Speed on a Road Trip | Physics Help for Beginners

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Homework Help Overview

The discussion revolves around calculating the average speed of a driver on a road trip, given specific average speeds for two segments of the trip. The problem involves variables A and B, which represent values that affect the average speeds, and participants are exploring how to interpret and solve the problem based on these values.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are attempting to clarify the values of A and B, questioning their roles in the problem. There are discussions about the time taken for each segment of the trip and how to convert total time from minutes to hours. Some participants express confusion about the formulation of the problem and the use of variables.

Discussion Status

The discussion is ongoing, with various interpretations being explored. Some participants have provided guidance on unit conversions and the need for clarity in the problem statement. There is no explicit consensus on the final average speed, and participants are still working through the calculations.

Contextual Notes

Participants note that the original problem statement may lack clarity regarding the definitions of A and B, and there is a mention of the potential confusion caused by the way the problem is presented. The discussion reflects a range of understanding among participants, with some being new to physics.

Lana Elcic
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Missing homework template. Originally posted in a different forum.
New to physics, so I'm looking for guidance and help. Here is the following question:

A=19
B=0

On a road trip, a driver achieved an average speed of (48.0+A) km/h for the first 54.0 km and an average speed of (43.0-B) km/h for the remaining 86.0 km. What was her average speed (in km/h) for the entire trip? Round your final answer to three significant figures.
 
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Step at a time: what's 48 + A? What's 43 - B?
 
67 and 43!
 
How long to travel the first 54 km?
 
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48.4 minutes?
 
And then 120 minutes (2 hrs) for second part?
 
Lana Elcic said:
120 minutes (2 hrs) for second part
Total time is what?
 
168.4 minutes
 
... and, the average for the total?
 
  • #10
Lana Elcic said:
New to physics, so I'm looking for guidance and help. Here is the following question:

A=19
B=0

On a road trip, a driver achieved an average speed of (48.0+A) km/h for the first 54.0 km and an average speed of (43.0-B) km/h for the remaining 86.0 km. What was her average speed (in km/h) for the entire trip? Round your final answer to three significant figures.
You can be new at physics but you have to understand how to put a problem. It seems you put numbers and letters at chance...
You should have formulated your problem in a better way. A and B are velocities? You have to specify it AND their unit of measures. Then I don't undestand why you put letters but then you substitute them with numbers. You want a numerical solution or one that depends on generic values A and B? Please don't think that we are looking at your mind or that we all have one only book which is the one you are studying in this moment :smile:

--
lightarrow
 
  • #11
Lana Elcic said:
(48.0+A) km/h for the first 54.0 km and an average speed of (43.0-B)
The original problem statement.
lightarrow said:
A and B are volocities?
 
  • #12
Bystander said:
The original problem statement.
Yes, but she can't write: "A = 19". It's not this the correct way to write data.

--
lightarrow
 
  • #13
lightarrow said:
Yes, but she can't write: "A = 19". It's not this the correct way to write data.

--
lightarrow
This is a very common way of writing problems in high-school litterature which is not wrong per se, but not very illuminating. The point you should note is that it says "(43+A) km/h" etc. As such, A itself must be dimensionless.
 
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  • #14
Orodruin said:
This is a very common way of writing problems in high-school litterature which is not wrong per se, but not very illuminating. The point you should note is that it says "(43+A) km/h" etc. As such, A itself must be dimensionless.
(43+A) km/h = 43 km/h + A km/h.

--
lightarrow
 
  • #15
lightarrow said:
(43+A) km/h = 43 km/h + A km/h.

--
lightarrow
Yes, which is fine unit wise since A is just a number. You will often find high-school assignments statements such as "let a car with the mass ##m## kg ..."
 
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  • #16
Orodruin said:
Yes, which is fine unit wise since A is just a number. You will often find high-school assignments statements such as "let a car with the mass ##m## kg ..."
Infact I would have written: "let a car with speed A km/h..."
Using numbers as you say seems very student-confusing for me, don't know if that's a good pedagogical way to teach physics problems (but are you talking of US high schools? in Italy it's not so).

--
lightarrow
 
  • #17
lightarrow said:
Infact I would have written: "let a car with speed A km/h..."
Using numbers as you say seems very student-confusing for me, don't know if that's a good pedagogical way to teach physics problems (but are you talking of US high schools? in Italy it's not so).

--
lightarrow
I would say that your way of writing is also bad form. A physicist would simply write "let the velocity be v". Since the velocity is unknown, it does not really matter which units you use and the natural thing to do is to include the dimension in the variable itself. Otherwise all of your physical relations such a F = ma depend on the units chosen for the corresponding (now dimensionless) quantities.

What you simply cannot do is to say "let the velocity be A km/h" and later claim that "A = 13 km/h" as this would make the velocity 13 km^2/h^2, which is not a velocity. This is just wrong.

But this is going away from the topic of the OP so we should drop it.
 
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  • #18
Orodruin said:
But this is going away from the topic of the OP so we should drop it.
Ok,
regards.

--
lightarrow
 
  • #19
Bystander said:
The original problem statement.
 
Last edited:
  • #20
lightarrow said:
You can be new at physics but you have to understand how to put a problem. It seems you put numbers and letters at chance...
You should have formulated your problem in a better way. A and B are velocities? You have to specify it AND their unit of measures. TheI don't undestand why you put letters but then you substitute them with numbers. You want a numerical solution or one that depends on generic values A and B? Please don't think that we are looking at your mind or that we all have one only book which is the one you are studying in this moment :smile:

--
lightarrow
I just copied and pasted it from my homework. The A and B are only used to individualize each student's problem. "Let A be the sum of the last four digits and let B be the last digit of your 8-digit student ID." I just didn't think to substitute it before posting.
 
  • #21
Bystander said:
... and, the average for the total?

Is it 42.1? I honestly don't know :cry::cry::cry:
 
  • #22
Bystander said:
The original problem statement.
Or is it 49.3? maybe 56.5?
 
  • #23
Lana Elcic said:
Or is it 49.3? maybe 56.5?
Item 1: Welcome to PF.
Item 2: Ignore the interruptions --- you were doing fine.
Item 3: Where were we?
Lana Elcic said:
168.4 minutes
You have been asked for average speed in what units?
 
  • #24
Bystander said:
Item 1: Welcome to PF.
Item 2: Ignore the interruptions --- you were doing fine.
Item 3: Where were we?

You have been asked for average speed in what units?
km/h
 
  • #25
You have total time in minutes. Need a conversion?
 
  • #26
Bystander said:
You have total time in minutes. Need a conversion?
So is it just 140 km / 2.48 hr = 56.452?
 
  • #27
It's my second day ever taking physics without even being introduced to the basic terms. I don't really understand why there's negativity.. I have to take this class for my major.. it's just introductory physics.
 
  • #28
168.4 min. = ?
Lana Elcic said:
2.48 hr = 56.452?
 
  • #29
Bystander said:
168.4 min. = ?
Doesnt 168.4 minutes convert to 2.48 hours?
 
  • #30
48.4/60 = ?
 

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