# Thread: work , energy and power

1. ## work , energy and power

I get question A but the rest i dont get, can someone give me a clear worked out answer to each question thanks a lot.

you walk up a 100m high mountain, which takes you 5 hours . walking on the flat consumes about 400w. you are hoping to lose some of your 70kg with all this excercise but on the way you eat four small chocolate bars, which according to the wrapper will each supply 1130kj.

A) calculate the energy you expend during the 5 hour walk, assuming that while you are climbing, your body works with an efficieny of only 15%?

B) calculate the energy that must be obtained from your body after the energy from the chocolate has been used?

C) if fat converts to energy at a rate of 38MJ-1 how much mass are you likely to lose?

2. oh my goodness 18 views and still no answer?

is it cause no one can be bothered to answer or you dont get' it' like me?

3. Originally Posted by hana_102
I get question A but the rest i dont get, can someone give me a clear worked out answer to each question thanks a lot.

you walk up a 100m high mountain, which takes you 5 hours . walking on the flat consumes about 400w. you are hoping to lose some of your 70kg with all this excercise but on the way you eat four small chocolate bars, which according to the wrapper will each supply 1130kj.

A) calculate the energy you expend during the 5 hour walk, assuming that while you are climbing, your body works with an efficieny of only 15%?

B) calculate the energy that must be obtained from your body after the energy from the chocolate has been used?

C) if fat converts to energy at a rate of 38MJ-1 how much mass are you likely to lose?
The problem I had with it is that the chocolate gives you more energy than you have expended so you don't lose any mass. Also your unit in C) is obscure. What is a $\displaystyle M/J$?

-Dan

4. Originally Posted by topsquark
The problem I had with it is that the chocolate gives you more energy than you have expended so you don't lose any mass. Also your unit in C) is obscure. What is a $\displaystyle M/J$?

-Dan

oh I missed out the kg-1 its 38mjkg-1

so question A and B are trick quiections? theres no answer for them ?

5. Originally Posted by hana_102
oh I missed out the kg-1 its 38mjkg-1

so question A and B are trick quiections? theres no answer for them ?
A quick note here about units.

A unit like Joules is named after a person so it is capitalized: "J," not "j." The same comment applies to Watts (W).

The unit meter has the designation "m." The prefix mega ($\displaystyle 1 \times 10^6$) has the designation M. Are you trying to say that fat converts to energy at a rate of $\displaystyle 38~MJ/kg = 3.8 \times 10^7~J/kg$. In other words you mean that 1 kg of fat, when converted, provides 38 MJ of energy?

The problem I have is that the answer for A is
$\displaystyle \frac{70~kg \cdot 9.8~m/s^2 \cdot 100~m}{0.15} = 457333~J = 457.333~kJ$

$\displaystyle 4 \times 1130~kJ = 4520~kJ$
which is vastly greater than the energy expended in A.

I wouldn't say it's a trick question, simply that someone made a typo somewhere.

-Dan

6. Originally Posted by topsquark
A quick note here about units.

A unit like Joules is named after a person so it is capitalized: "J," not "j." The same comment applies to Watts (W).

The unit meter has the designation "m." The prefix mega ($\displaystyle 1 \times 10^6$) has the designation M. Are you trying to say that fat converts to energy at a rate of $\displaystyle 38~MJ/kg = 3.8 \times 10^7~J/kg$. In other words you mean that 1 kg of fat, when converted, provides 38 MJ of energy?

The problem I have is that the answer for A is
$\displaystyle \frac{70~kg \cdot 9.8~m/s^2 \cdot 100~m}{0.15} = 457333~J = 457.333~kJ$

$\displaystyle 4 \times 1130~kJ = 4520~kJ$