Homework Help: Energy Question - Burning off Breakfast

1. Sep 9, 2004

coglon

Energy Question -- Burning off Breakfast

Im not to good at physics to begin with, and Ive been having alot of trouble with this question:
"How many steps does it take to wear off your breakfast?"
First of all this question doesnt give you any starting points with variables and such and I really have no clue what kind of equation to use.

I am using Shredded Wheat & Bran as my breakfast (30g per serving (160ml)) without milk and this contains 450kJ from the back of the box nutritional facts.
I assumed that each step would take approx. 1 meter in length and that the person burning off the breakfast weights 70kg. I dont know if this info is relavent to the question but im kinda stuck.

I thought if I could figure out how many joules were burn each step by this person I could divide that by the total joules of breakfast and figure out how many steps it will take

I would think that W=Fd would be the starting equation but I dont have a variable with Newtons

2. Sep 10, 2004

chroot

Staff Emeritus
If you could figure out how many joules are expended in each step, certainly, you could figure it out. The problem is there is really no way to determine the amount of energy expended in each step except by experimenting.

When you walk on level ground, you're not doing any work against gravity. You're only doing work against friction -- the friction of your shoes against the pavement, the friction of your joints moving, the "friction" of your individual muscle fibers contracting and relaxing, and so on. The human body is an extremely complex machine, and it's just not possible to figure out how much energy a person expends in a step by using a simple formula.

I did a little Googling, and there seems to be a consensus that a 150-pound person walking at 3 mph burns about 4 kilocalories per minute. (A kilocalorie is the same as a Calorie you see on nutrition information.)

It looks like it takes about a mile and a half:

- Warren

3. Sep 10, 2004

Sirus

You definitely need to approximate here, but I think you can solve this problem in another way. When you lift your body to the next step, assuming no friction of joints and the like, you must overcome the gravitational force acting on your body (which is known in this problem). In theory, the force you apply to the step must only be minutely larger than the gravitational force in order for you to move upward. So if you assume applied force equals gravitational force for each step and solve the problem that way, you would be making a decent approximation of the number of steps required to burn your breakfast in a frictionless environment and assuming your body only uses energy to move.

4. Sep 10, 2004

coglon

thank you for the replys, well it came down to deadline so I assume there was no friction what so ever and i ended up with something like this for a equation:

W= F*d
W= (9.81m/s^2*70)*d
Substituted joules from breakfast as work:
450kj=(9.81m/s^2*70)*d
(450kJ)/(9.81m/s^2*70)=d
d=655m <--going off memory could be wrong but thats the process i used... do you guys think this is right??
so 655 steps to wear off

5. Sep 11, 2004

chroot

Staff Emeritus
If you walk along level ground, you have no net change in elevation, and thus there is no net work done against gravity.

The bottom line is that the number of Calories burned per step is not calculable from first principles, because the human body is much too complex.

- Warren