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.

Short answer

(a) The duration for which you can play tennis is$30.3min$ .

(b) Yes, it seems like a long time.

Step-by-step solution

Step 1: Power

Power is scalar quantity which is defined how fast the energy is being consumed by the system.

Mathematically,

$P=\frac{E}{t}$……………….. (1.1)

Here, E is the amount of energy consumed, and t is the time.

The time which you can play tennis

(a)

The time can be calculated using equation (1.1).

Rearranging equation (1.1) in order to get an expression for time.

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

Here, E is the energy of the tennis player$\left(E=800\text{kJ}\right)$ , and P is the power consumed while playing tennis$\left(P=440\text{W}\right)$ .

Putting all known values,

$\begin{array}{rcl}t& =& \frac{800\text{kJ}}{440\text{W}}\\ & =& \frac{\left(800\text{kJ}\right)\times \left(\frac{1000\text{J}}{1\text{kJ}}\right)}{440\text{W}}\\ & =& 1818.18\text{sec}\times \left(\frac{1\text{min}}{60\text{sec}}\right)\\ & =& 30.3\text{min}\\ & & \end{array}$

Therefore, the duration for which you can play tennis is$30.3min$ .

Exercise is necessary

(b)

Yes, this is surprisingly long. Exercise is required to burn more energy than is consumed.