Kinetics

Newton's Second Law of Motion describes relation between a cause (external force) and an effect (acceleration). When a human body accelerates, we know that a non-zero resultant external force must be acting on it. If a non-zero resultant external force ΣF (the sum of all external forces acting on the body) acts on a human body with the mass m, we know that the body must accelerate. This fact can be mathematically expressed as:

where a is acceleration.

---

Every time a human body, or any object in sport, decreases its velocity, increases its velocity, or changes the direction of its motion, it moves with non-zero acceleration. Resultant external force is the cause of this acceleration³⁹.

---

Let us have a look, for example, at weight training exercises. What forces must act on a 30kg barbell during bench press exercise? There is gravitational force with downward direction and reaction force exerted by our arms in the opposite direction. The resultant external vertical force (black line in Fig. 11) is the difference between these two forces.

Please note that the resultant force is greater at the beginning of the motion when we start moving the barbell and hence we are accelerating. Later we move the barbell with little acceleration and therefore the resultant force is lesser. In the opposite direction, when we are stopping the barbell, a breaking force must be acting and therefore the barbell decelerates (negative force). If we only hold the barbell in our hands, we must exert approximate force of 294,3 N (force = mass x gravitational acceleration). If, however, we accelerate, we must exert force greater that the weight of the barbell, as we can see in Fig. 13⁴⁰ (force = mass x (gravitational acceleration + the barbell acceleration)).

If we compare resultant forces that must act on the barbell in vertical and horizontal direction, we find out that horizontal forces are lesser than vertical forces (Fig. 11). This is, apart from other things, caused by the fact that there is no gravitational force in horizontal direction and therefore in this direction even a weak force is enough to regulate the motion of the barbell. In Fig. 11 we can see that anteroposterior forces (blue) are greater than mediolateral forces (grey).

Figure 11 Dependence of resulting external force on time in bench press exercise with 29 kg barbell. Back line: vertical component of the force; Blue line: anteroposterior component of the force; Grey line: mediolateral component of the force. The range of motion is from the lowest position of the barbell to its highest point achieved during the exercise.

Let us have a look at another example. What is happening when an athlete is running with constant velocity v along a left curve with the radius r? Is he accelerating? Yes, he is, because he is changing the direction of his velocity! Such acceleration is called centripetal a_d and its magnitude is calculated as:

What is the origin of forces which cause his centripetal acceleration in horizontal direction? Let us imagine for a second that the same athlete is running on an ice rink. That would probably end in disaster. In the curve he would instantly slip and fall. Friction is the external horizontal force causing changes to direction and the resultant centripetal acceleration of the athlete.

³⁹ Start of sprint for 100 metres is a good example of acceleration in sport.Zpět

⁴⁰ In Fig. 13 we can see the resultant force. In reality, however, we act on the barbell with the force equal to reaction force which is greater by the gravitational force of 294,3 N throughout the whole line of the dependence of force on time. However, we need to know the resultant force in order to specify the acceleration of the barbell. Both forces act on the barbell – reaction force and gravitational force.Zpět