Applying force to an object

Frequently in sport the body is the vehicle used for applying force to an object. This could be the swing of a racket or bat applying force to a ball, or the movement of a foot to kick a ball, or even the use of an athlete’s body to tackle their opposition. The body does this through the generation of forces by their muscles creating movement of the limb or body that makes contact with the external object.

In order to generate the maximal force, the body needs to use its largest muscles. These are the muscles of the lower body and torso. Even when using the upper body to make contact with the external object, the body will often generate the force from the lower half. If we take a look at shot put for example. The video below is of the Olympic record being set. If we focus on the technique, you can see that each athlete who throws, bends their legs and generates their force from their legs. They then add force from their torso, as they twist their body, before the final push with the arms. In the video, they also add centrifugal force generated by the spin.

The body can also help to generate force, by using gravity and momentum from movement and transferring the force into the object. This can be seen in a power shot in football, where the athlete will take a run up, to generate momentum. When they then plant their foot next to the ball before kicking it, they will bend the planted leg, drop their centre of gravity and then use the muscles in their torso and legs to generate the force of the kick, which will include a swinging motion at the hips and twisting at the torso. Such technique, can be seen in the video below as the athlete’s use the “knuckle ball” shooting technique, to not only apply large forces to the ball, but also to cause the ball to move around unpredictably in the air.

The last example of applying force to an object is the rugby tackle. Here the athlete seeks to apply a force to their opposing player in order to bring them to the ground. There are various tackling techniques that can be used. The first technique, requires the tackler to step forwards towards their opponent and drive with their legs in a forward and upwards direction. The aim is to stop the athlete’s momentum, and lift their centre of gravity to make the athlete unstable, before putting them on their back. This tackling technique however, relies on the tackler being able to produce a very large force on their opponent.

For a smaller tackler, however, this type of tackle is not safe, as the forces from the opposition will cause him to fall off the tackle. Instead, such tacklers seek to apply forces in other directions, such as sideways, or downwards. A smaller force is then required to bring the opponent to the ground because the tackler is aiming to cause the runner to loose balance, rather than counter act the momentum of the athlete. In fact, if done well the momentum of the runner should assist the tackler.

These are just a few examples of athlete’s using parts of their body in applying force to an object. We could look at angular momentum around joints and how the longer the lever the faster the movement at the end, resulting in greater forces being applied to the object (why we kick with our feet and not our knees or hit with our hands and not our elbows), but this might be more depth than you need.

Further Reading