Kinematics
Velocity
When we say that somebody is slow or fast we are actually describing his/her velocity or, to be more precise, his/her vector of velocity. In biomechanics one of our tasks is to quantitatively describe velocity.
Mean Spee
Scalar absolute value of velocity is speed. Mean speed v is the distance Δs divided by time Δt taken to cover the distance:
The SI unit is metre per second, i.e. the unit of distance divided by the unit of time (m/s). Another often used unit is kilometre per hour (km/h). Mean speed is important in describing performance in many sport events. In the events where athletes must cover the same distance their speed is the direct measure of their success. In cross country skiing, for example, the winner covers the given distance in shortest possible time. The mean speed of an athlete is the distance he/she covers divided by his/her final time. The final mean speed of the winner will always be higher than that of all other competitors.
Mean speed, however, does not say much about the actual course of the race. It gives no information about how fast the racers covered individual parts of the track. We know nothing about their maximum speed and about those sections of the track where racers speeded up or slowed down. Coaches usually need to know how fast their charge was right after the start or in the final sprint.
Marathon tactics is a good example of making use of mean speed. Each runner can choose a pacemaker to run after. In 2011 Prague marathon, for example, there was a pacemaker to make the final time of 03:00:00 (hrs:min:s) according to Table 1.
Table 1 Split times and mean speeds of pacemaker in sections 1-5, 10-15, 20-25, 30-35 and 40-marathon, to make the final time of 3 hours.
| Distance covered (m) | Split time (hod:min:s) | Section mean speed (m/s) |
| 1 000 | 4:28 | 3,73 |
| 5 000 | 22:20 | |
| 10 000 | 43:50 | 3,94 |
| 15 000 | 01:05:00 | |
| 20 000 | 01:25:10 | 3,90 |
| 25 000 | 01:47:30 | |
| 30 000 | 02:09:00 | 3,97 |
| 35 000 | 02:30:00 | |
| 40 000 | 02:51:30 | 4,17 |
| 42 125 | 03:00 |
The values of mean speed often vary during the course of the race and if a racer knows them, he/she can subsequently correct his/her training details. If the length of the sections in which we measure time approaches zero, the resultant value is instantaneous velocity.
Instantaneous velocity is the velocity of a moving object at a particular instant of time. It is the velocity attained by the object in a very short period of time (approaching zero).
Displacement Velocity
Displacement velocity can be defined as motion over a given interval of time, i.e. the displacement divided by the length of the time interval. As displacement is a vector, displacement velocity is also a vector. Sometimes we need to know individual components of displacement velocity vector. Just as we resolved force we can also resolve displacement velocity vector.
In swimming events we are more interested in mean speed. On the other hand in downhill skiing, for example, we are not that interested in mean speed but rather in displacement velocity, i.e. how fast skiers moved from start to finish, regardless the actual speed of going around gate poles. Even a skier with lower mean speed can beat his/her opponents if he/she chooses better trajectory.
Importance of Speed
There are many sport events in which the winner is the competitor with the highest mean speed. Speed is also important in sport events where it is only one of the factors influencing the overall performance. Let us have a look at tennis serve. After a good serve the ball flies with the speed of 100 - 200 km/h and it is really difficult to come up with an equally good return. Why? The faster the ball flies the less time the opponent has to move. For example Ivan Karlovič is able to serve with the record speed of 251 km/h (70 m/s20). The distance between the opponents is about 24 m. How much time does the opponent have to react to Ivan Karlovič’ serve?
time = displacement / speed
time = 0.34 s
The opponent has only about 0.3 second for a motor task of precisely returning the ball. We can see that in tennis the speed of the ball determines the time players have to manage a stroke or to move to a desired position. In the same way in sports such as football, floorball, hockey, and handball the speed of a flying projectile is important for goal keepers because it determines the amount of time they have to catch it.
Speed is also a positive factor of performance in long jump, triple jump, high jump, and ski jumping. Gymnasts need high speed in vault so that they can manage the given number of rotations.
20 This record speed of serve was measured in 2011 during the Davis Cup match between Chroatia and Germany.Zpět