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

AI Thread Summary
The discussion revolves around calculating the average speed of a road trip using given variables A and B, which represent adjustments to the average speeds for two segments of the trip. Participants emphasize the importance of clearly defining variables and their units in physics problems, noting that A and B should be dimensionless for proper calculations. The conversation includes step-by-step calculations, leading to a total travel time of 168.4 minutes, which converts to approximately 2.8066 hours. Ultimately, the average speed is calculated to be around 49.9 km/h when rounded to three significant figures. Clarity in problem formulation and unit conversion is highlighted as crucial for understanding physics concepts.
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 = ?
 
  • #31
.807.

Why are you dividing this? I'm just trying to fully understand the problem for the test! :)
 
  • #32
Lana Elcic said:
.807.

Why are you dividing this? I'm just trying to fully understand the problem for the test! :)
Bystander said:
48.4/60 = ?
 
  • #33
2 hrs is 120 min, 48.4 min. is .8xxx hrs. --- ?
 
  • #34
Bystander said:
2 hrs is 120 min, 48.4 min. is .8xxx hrs. --- ?

.8066
 
  • #35
Total time in hours is then what?
 
  • #36
Bystander said:
Total time in hours is then what?
2.8066?
 
  • #37
Lana Elcic said:
So is it just 140 km / 2.48 hr = 56.452?
Nope. Right idea.
Lana Elcic said:
Doesnt 168.4 minutes convert to 2.48 hours?
Nope.
Lana Elcic said:
2.8066?
... and the answer is ...?

Sorry about all the distractions. I know it's enough to rattle anyone. Couple or three things to remember: 1) try to post in the HW forum (for physics questions, anyway); 2) one step at a time; 3) re-read the question to be certain you're answering what's been asked; 4) pay attention to details, particularly when converting units (minutes to hours is something public schools haven't done correctly for ~ 20 years); 5) PF busy season has just started the last week or two, and there can be a lot of help that just isn't.
 
  • #38
Bystander said:
Nope. Right idea.

Nope.

... and the answer is ...?

Sorry about all the distractions. I know it's enough to rattle anyone. Couple or three things to remember: 1) try to post in the HW forum (for physics questions, anyway); 2) one step at a time; 3) re-read the question to be certain you're answering what's been asked; 4) pay attention to details, particularly when converting units (minutes to hours is something public schools haven't done correctly for ~ 20 years); 5) PF busy season has just started the last week or two, and there can be a lot of help that just isn't.

It's okay I understand. Just trying to focus. Is the answer 49.882?
 
  • #39
Lana Elcic said:
Round your final answer to three significant figures.
You're in business.
 
  • #40
Bystander said:
You're in business.
So 49.9!
Yayyyyyy!
 
  • #41
Thread closed temporarily for Moderation...
 

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