Homework Help: A Joule's experiment question

1. Jan 21, 2010

kavamo

1. The problem statement, all variables and given/known data

An experiment is conducted with a basic Joule apparatus, where a mass is allowed to descend by 1.23 m and rotate paddles within an insulated container of water. There are several different sizes of descending masses to choose among. If the investigator wishes to deliver 1.00 kcal to the water within the insulated container after 30.0 descents, what descending mass value should be used? Let g = 9.80 m/s2. (need answer is in kg)

2. Relevant equations

U=mg(delta)h

3. The attempt at a solution

1kcal=m(9.8)*30*1.23
4186 =m(361.62)
4186/361.62=m
11.576=m

Did I do this correctly?

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 21, 2010

doive

^ assuming your conversion of kcal to joules is correct that seems like a model solution to me.

3. Jan 21, 2010

mgb_phys

1 Kcal = 4186 joules
As you say potential energy = mgh
Rememebr to put the units in:

E = m g h
4186 J = m kg * 9.81 m/s^2 * (30*1.23)m
Check units, J = N m = kg m/s^2 m

so m = 4186 / (9.8 * 30 * 1.23) = 11.57kg

4. Jan 21, 2010

kavamo

Thanks for the reminder!