# help with some math problems, introduced in physics class

• 09-30-2009
Akkernight
help with some math problems, introduced in physics class
Hello!

I need some problems solved for physics class which is soon, so I'm taking my chances here. If no one answers before, I still want to learn how it's done :D
So here we go.

Translation from Danish:

On the motor of a turn-bench it says that it's max horsepower if 2.0 HK. When in use at work it uses 1.2 HK. The work takes 5 minutes.

a. How much Joules does the motor use?
b. How much kWh does the motor use?
c. What's the cost of the energy, when 1 kWh costs 1,50 kr.?

I DO NOT just want answers, I want to learn and learn how it's done :D So if anyone has the time and feels like helping out, please do so! It WILL be much appreciated!
• 09-30-2009
stevesmithx
What have done so far?
Anyway here's a clue for solving a) part
1.Find out which physical quantity has Horsepower as its unit.
2.Find out which physical quantity has Joules as its unit.
3.Find out which physical quantity has minutes as its unit!.;)
4.Find out the formula relating the above three quantities.
5.Substitute for known quantities and find the unknown part.

Hint: Use google if you are stuck in any of the above steps.
• 09-30-2009
Akkernight
a.

δE=P∙δt
δE=895∙300
δE=268500 J

b.

kWh=0,895/(5 mins)=(0,895∙12)/(5∙12)=10,74kW/1h=10,74kWh

c.

1 kWh=1,59 kr.
10,74 kWh=1,59∙10,74=17,08kr

---

Is this correct? I had to figure out the algorithms/formulas myself, and some of my class mates ain't getting the same result, they claim to be using some formula told by the teacher, yet I can't see why my result should be wrong.

EDIT: ',' means the same as '.' just that it's opposite in Scandinavia and other countries, 1.8 == 1,8
• 09-30-2009
stevesmithx
a) part apart from a minor approximation is Ok i think.[746*1.2=895.2~895] But i couldn't make heads or tails of your solution for the b) part(With all that use of ','s and '.'s which is very peculiar to me). Either case, look up here.
• 09-30-2009
nonoob
A quick search found this:
1 horsepower hour = 2684519.5368856 joules
1 hourspower = 0.7456 kilowatts
That should be enough information.
• 09-30-2009
whiteflags
How much is 1.2 jigawatts?
• 09-30-2009
hk_mp5kpdw
Quote:

Originally Posted by whiteflags
How much is 1.2 jigawatts?

I thought it was 1.21 gigawatts? Jigawatts don't exist... except maybe in Jay-Z's world.
• 09-30-2009
NeonBlack
Quote:

Originally Posted by hk_mp5kpdw
I thought it was 1.21 gigawatts? Jigawatts don't exist... except maybe in Jay-Z's world.

When you're powering a magnetic flux capacitor, they're called "jigawatts".
• 09-30-2009
Neo1
Quote:

Originally Posted by whiteflags
How much is 1.2 jigawatts?

Same as the average lightning bolt obviously.
• 09-30-2009
brewbuck
Questions a and b are the same thing. Joules and kWh are both units of energy or work. It's just a difference of unit (like feet vs. meters).

If you take a power and multiply it by a time, you get a quantity with units of energy. But to get a meaningful unit, you need to first convert the horsepower to a more standard unit: watts. The time should also be converted to seconds instead of minutes. The product of these two has unit watt-second, which is the same thing as a joule.

To convert from joules to kWh, divide by 3600000.

The third part is simple given the solution to the second part.