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

[Lana Elcic]

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.

 

[Bystander]

Step at a time: what's 48 + A? What's 43 - B?

 

[Lana Elcic]

67 and 43!

 

[Bystander]

How long to travel the first 54 km?

 

[Lana Elcic]

48.4 minutes?

 

[Lana Elcic]

And then 120 minutes (2 hrs) for second part?

 

[Bystander]

120 minutes (2 hrs) for second part

Total time is what?

 

[Lana Elcic]

168.4 minutes

 

[Bystander]

... and, the average for the total?

 

[lightarrow]

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

--
lightarrow

 

[Bystander]

(48.0+A) km/h for the first 54.0 km and an average speed of (43.0-B)

The original problem statement.

A and B are volocities?

 

[lightarrow]

The original problem statement.

Yes, but she can't write: "A = 19". It's not this the correct way to write data.

--
lightarrow

 

[Orodruin]

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.

 

[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.

(43+A) km/h = 43 km/h + A km/h.

--
lightarrow

 

[Orodruin]

(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 ..."

 

[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 ..."

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

 

[Orodruin]

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.

 

[lightarrow]

But this is going away from the topic of the OP so we should drop it.

Ok,
regards.

--
lightarrow

 

[Lana Elcic]

The original problem statement.

 

[Lana Elcic]

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

--
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.

 

[Lana Elcic]

... and, the average for the total?

Is it 42.1? I honestly don't know

 

[Lana Elcic]

The original problem statement.

Or is it 49.3? maybe 56.5?

 

[Bystander]

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?

168.4 minutes

You have been asked for average speed in what units?

 

[Lana Elcic]

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

 

[Bystander]

You have total time in minutes. Need a conversion?

 

[Lana Elcic]

You have total time in minutes. Need a conversion?

So is it just 140 km / 2.48 hr = 56.452?

 

[Lana Elcic]

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.

 

[Bystander]

168.4 min. = ?

2.48 hr = 56.452?

 

[Lana Elcic]

168.4 min. = ?

Doesnt 168.4 minutes convert to 2.48 hours?

 

[Bystander]

48.4/60 = ?

 

[Lana Elcic]

.807.

Why are you dividing this? I'm just trying to fully understand the problem for the test! :)

 

[Lana Elcic]

.807.

Why are you dividing this? I'm just trying to fully understand the problem for the test! :)

48.4/60 = ?

 

[Bystander]

2 hrs is 120 min, 48.4 min. is .8xxx hrs. --- ?

 

[Lana Elcic]

2 hrs is 120 min, 48.4 min. is .8xxx hrs. --- ?

.8066

 

[Bystander]

Total time in hours is then what?