# Average Speed -- Help please

1. Aug 29, 2015

### Lana Elcic

• 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.

2. Aug 29, 2015

### Bystander

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

3. Aug 29, 2015

### Lana Elcic

67 and 43!

4. Aug 29, 2015

### Bystander

How long to travel the first 54 km?

5. Aug 29, 2015

### Lana Elcic

48.4 minutes???

6. Aug 29, 2015

### Lana Elcic

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

7. Aug 29, 2015

### Bystander

Total time is what?

8. Aug 29, 2015

### Lana Elcic

168.4 minutes

9. Aug 29, 2015

### Bystander

... and, the average for the total?

10. Aug 29, 2015

### lightarrow

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

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lightarrow

11. Aug 29, 2015

### Bystander

The original problem statement.

12. Aug 29, 2015

### lightarrow

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

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lightarrow

13. Aug 29, 2015

### Orodruin

Staff Emeritus
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.

14. Aug 29, 2015

### lightarrow

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

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lightarrow

15. Aug 29, 2015

### Orodruin

Staff Emeritus
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 ..."

16. Aug 29, 2015

### lightarrow

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).

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lightarrow

17. Aug 29, 2015

### Orodruin

Staff Emeritus
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.

18. Aug 29, 2015

### lightarrow

Ok,
regards.

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lightarrow

19. Aug 29, 2015

### Lana Elcic

Last edited: Aug 29, 2015
20. Aug 29, 2015

### Lana Elcic

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.