Question

. (a) How long can you play tennis on the 800 kJ (about 200 kcal) of energy in a candy bar? (b) Does this seem like a long time? Discuss why exercise is necessary but may not be sufficient to cause a person to lose weight. Answer from book:a) 30 or 32 minb) exercise does not burn calories, but requires a long time to burn off minor amounts of food.

Answer 1

(a). The power of the candy bar is,

$P=440\text{ W}$

The time taken to play on 800 kJ energy of the candy bar is,

$t=\frac{E}{P}$

where E is the energy.

Substituting the known values,

$\begin{gathered} t=\frac{800\times10^3}{440} \\ t=1.818\times10^3\text{ s} \\ t=1818\text{ seconds} \end{gathered}$

As 1 minute is equal to 60 seconds,

Thus,

$\begin{gathered} t=1818\text{ seconds} \\ t=\frac{1818}{60} \\ t=30.3\text{ min} \end{gathered}$

Thus, the time taken to play tennis on the 800 kJ energy is 30.3 minutes.

(b). By doing the exercise, the process of digestion of food inside our body increases. Thus, the exercise does not helps us to burn the calories. But it helps us to diggest the heavy meal like candy bar easily.

The time taken to digest the canndy bar or to utilise its energy is large because it takes a lot of time to burn small amount of food and make it digest quickly.