Change in chemical energy of rocket

[isukatphysics69]

Homework Statement

Homework Equations

The Attempt at a Solution

19800*.35 = 6930J

 

[isukatphysics69]

really need help here

 

[haruspex]

19800*.35 = 6930J

What is the relationship between the energy expended, the useful work done, and the efficiency?

 

[isukatphysics69]

What is the relationship between the energy expended, the useful work done, and the efficiency?

energy expended divided by work is efficiency?

 

[haruspex]

energy expended divided by work is efficiency?

Would that tend to produce a value > 1 or < 1?

 

[isukatphysics69]

Would that tend to produce a value > 1 or < 1?

it depends tho

 

[haruspex]

it depends tho

Will the work expended be more or less than the useful work done?
Where might the work expended go?

 

[isukatphysics69]

ok so since there is wasted energy then the work expended is more than the useful work done

 

[haruspex]

ok so since there is wasted energy then the work expended is more than the useful work done

So what would be a sensible way to define efficiency?

 

[isukatphysics69]

So what would be a sensible way to define efficiency?

so 35/100 particles of rocketfuel are usefull
so that's 35% so 35% of the total energy is 35% of 19800

 

[haruspex]

Right, so what should you do with the 19800J you calculated?

 

[isukatphysics69]

Right, so what should you do with the 19800J you calculated?

so total energy expended was 19800 jules. only 35 percent of that is used efficiently so out of the total 19800 6930 was put to use that's what i thought

 

[haruspex]

so total energy expended was 19800 jules

No. How did you calculate the 19800 J? Go back and check.

 

[isukatphysics69]

No. How did you calculate the 19800 J? Go back and check.

oh yea ok so the change in chemical potential energy was 19800 joules

 

[haruspex]

oh yea ok so the change in chemical potential energy was 19800 joules

There are the following quantities of work...

Useful work done
Total work done
19800J
GPE gained
KE gained
Chemical PE lost
Work lost to heat, sound etc.

What are all the relationships between these?

 

[isukatphysics69]

There are the following quantities of work...

Useful work done
Total work done
19800J
GPE gained
KE gained
Chemical PE lost
Work lost to heat, sound etc.

What are all the relationships between these?

They all have to do with energy

 

[haruspex]

They all have to do with energy

No, I mean what equations relate them. Some are equal, some add up to equal others...

 

[isukatphysics69]

There are the following quantities of work...What are all the relationships between these?

Total work done = GPE gained + Work lost to heat, sound etc. + KE gained
Chemical PE lost = 19800J Since it is the change in PE
Useful work done = KE gained​

 

[haruspex]

Total work done = GPE gained + Work lost to heat, sound etc. + KE gained

Yes.

Chemical PE lost = 19800J Since it is the change in PE

No. Look at how you calculated 19800J in post #1. I see nothing about chemical PE there.

Useful work done = KE gained

You left out GPE gained.

 

[isukatphysics69]

Yes.

No. Look at how you calculated 19800J in post #1. I see nothing about chemical PE there.

You left out GPE gained.

ok there is a mistake there both sides should be multiplied by negative 1 so CPE final - CPE initial will be ΔCPE

 

[haruspex]

ok there is a mistake there both sides should be multiplied by negative 1 so CPE final - CPE initial will be ΔCPE

No.
Please try to address my points in post #19.