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Work-energy principles in modern shot-putting

This is an excerpt from Biomechanics of Sport and Exercise 4th Edition With Web Resource-Loose-Leaf Edition by Peter M. McGinnis.

Concept Application

The official distance of a shot-put is measured along a radial line from the inside edge of the circle to the nearest mark made by the shot where it first strikes the landing area. This distance is determined by the velocity of the shot at release, the height of the shot at release, the projection angle of the shot at release, and how far the shot was in front of or behind the inside edge of the throwing circle at release. The release velocity of the shot has the greatest effect on the distance of the throw. The shot's mass is constant, so the kinetic energy of the shot at release has the greatest effect on the distance of the throw.

The kinetic energy of the shot at release is produced by the work done by the athlete on the shot during the put. Remember, work done by a force is given by equation 4.2,

and net work done by nonconservative forces (forces other than gravity and strain forces) is given by equation 4.9,

Thus the average force exerted on the shot by the athlete, multiplied by the displacement of the shot along the line of action of this force, determines the changes in potential and kinetic energies of the shot at the end of the putting action. A larger force and displacement create a larger release velocity of the shot.

At the 16th IAAF World Championships in Athletics in 2017, sport scientists measured a variety of biomechanical parameters of each athlete during the men's and women's shot-put competitions (Dinsdale, Thomas, and Bissas 2017b, 2017c). The resultant displacement of the shot during the putting action could be determined from these biomechanical measurements for the best put by each shot-putter in the final. In the men's shot-put competition, Tomas Walsh won with a distance of 22.03 m. Of the 12 competitors in the final, 10 were taller than Walsh. Even with this height disadvantage, only 3 of the 12 competitors produced a longer resultant displacement of the shot during their putting actions. In the women's shot-put competition, Lijiao Gong won with a distance of 19.94 m. Of the 12 women competitors in the final, 9 were taller than Gong. Even with this height disadvantage, only 2 of the 12 competitors produced a longer resultant displacement of the shot during their putting actions. Gong's resultant displacement of the shot was only 6 cm less than that of the athlete with the longest displacement. It appears that the length of the resultant displacement of the shot during the putting action greatly affects the distance of the put. The work-energy principle supports this conclusion.

In this example, we do not know if the force the shot-putters applied to the shot during their putting action was along the line of the shot's resultant displacement. Remember that work done by a force is the product of the force and displacement of the object during the force application along the line of action of the force. Work can also be computed as force along the line of the displacement times the displacement. Another reason for the winning performances of Tomas Walsh and Lijiao Gong may be that the forces they applied to the shot were more effective because they aligned more closely with the displacement of the shot during their puts. The long displacement combined with the alignment of the force with this displacement produced large changes in energy during Walsh's and Gong's shot-puts.

The relationship between work and energy is one of the most important concepts in biomechanics. During activities when you want to increase the kinetic energy of an object or yourself, such as throwing, swinging, striking, or jumping, the most effective techniques use movements that allow force to be applied through a long displacement while maintaining the alignment of the force with the motion direction throughout the movement. At the elite level, small increases in displacement may be responsible for greater changes in kinetic energy and thus faster throws, swings, etc.

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