What Force Must a Soccer Player Exert to Reach Top Speed?

1. To solve an integrated concept problem, we must first identify the physical principles involved and identify the chapters in which they are found. Part (a) of this example considers acceleration along a straight line. This is a topic of kinematics. Part (b) deals with force, a topic of dynamics found in this chapter.
2. The following solutions to each part of the example illustrate how the specific problem-solving strategies are applied. These involve identifying knowns and unknowns, checking to see if the answer is reasonable, and so forth.

We are given the initial and final velocities (zero and 8.00 m/s forward); thus, the change in velocity is $\mathrm{\Delta }v=\mathrm{8.00\; m/s}$. We are given the elapsed time, and so $\mathrm{\Delta }t=\mathrm{2.50\; s}$. The unknown is acceleration, which can be found from its definition:

Substituting the known values yields

This is an attainable acceleration for an athlete in good condition.

Here we are asked to find the average force the player exerts backward to achieve this forward acceleration. Neglecting air resistance, this would be equal in magnitude to the net external force on the player, since this force causes his acceleration. Since we now know the player’s acceleration and are given his mass, we can use Newton’s second law to find the force exerted. That is,

Substituting the known values of $m$ and $a$ gives

This is about 50 pounds, a reasonable average force.

This worked example illustrates how to apply problem-solving strategies to situations that include topics from different chapters. The first step is to identify the physical principles involved in the problem. The second step is to solve for the unknown using familiar problem-solving strategies. These strategies are found throughout the text, and many worked examples show how to use them for single topics. You will find these techniques for integrated concept problems useful in applications of physics outside of a physics course, such as in your profession, in other science disciplines, and in everyday life. The following problems will build your skills in the broad application of physical principles.