Measurement in sport – the long and the short of it
Box 3 | Physics in sport: Forces on an athlete
A force can be defined as a push or a pull. When running or walking we use the energy in our leg muscles to push backwards on the ground with one foot while the other foot is moving forward. Forces always come in pairs called action-reaction pairs. So, when we push backwards on the ground, the ground also pushes forward on us.
Sir Isaac Newton first proposed this idea in the 1600s. He said that for every force (action) there is an equal force (reaction), which acts in the opposite direction. Action-reaction pairs of forces associated with an athlete are due to gravity, friction with the ground and air resistance.
Ground reaction force and shock absorbency
Runners move across a surface by a series of long strides. Each time a foot lands, the ground exerts a force on it, which is then distributed (absorbed) by the runner's body. Considerable damage can occur to the runner's ankles and knees unless the force is reduced. Elastic playing surfaces can absorb some of the force; so can running shoes with elastic foam layers in the soles.
An elastic playing surface such as grass feels springy to run on and produces fewer injuries than more rigid surfaces such as concrete. But the time spent rebounding is higher on a springy surface, slowing the runner down. The best surface for athletics is one that is absorbent enough to limit injuries but firm enough to give athletes the best chance of achieving optimum results.
Frictional forces
The force of friction applies when you move any two surfaces against each other. Friction works in a direction opposite to the direction of motion. The size of the frictional force depends on the force pushing the two surfaces together and their roughness.
Playing surfaces and the soles of running shoes must provide sufficient friction to ensure that runners do not slip. However, too much friction between surfaces and shoes can lead to ankle and knee injuries.
Measuring friction
The friction between two surfaces before they move is called static friction. Once the two surfaces are on the move it is easier to keep them moving and the value of the frictional force becomes less. This force is known as sliding friction.
The friction between surfaces can be compared by calculating the coefficient of friction. This is the frictional force in newtons divided by the weight of the object in newtons. It can be calculated for both sliding and static friction. Running tracks used for international competitions must have a minimum coefficient of sliding friction of 0.5 under wet conditions (when the surface is most slippery). This standard has been set to help prevent athletes from slipping unduly.
Physics in field athletics
Field athletics consists of four throwing events (discus, javelin, shot and hammer) and four jumping events (high jump, pole vault, long jump and triple jump).
All throws and jumps can be divided into four stages: the approach, the launch, the flight and the impact. The success of a throw or a jump will be affected by several factors. At the launch stage, the speed, angle and height of take-off will all influence the outcome. During the flight stage other factors come into play such as the shape of the object, the speed and direction of the wind, and the force of gravity.
An athlete strives to perfect a throwing technique by giving the object the greatest possible speed at the moment of release. Other factors such as the angle of release and the height of release also affect the distance travelled by the object.
Jumping
Another factor to consider in jumping events is the athlete's centre of mass (or centre of gravity). This is the point around which the total mass of the athlete is evenly distributed. Its exact position varies from person to person depending on build, and it shifts as you move.
The best jumping technique will be one that allows the jumper to keep their centre of mass as low as is consistent with clearing the bar. This allows the athlete to achieve a higher jump using less energy, because more energy is needed to raise the centre of mass higher.
In high jumping, a technique known as the Fosbury flop, introduced in the 1960s, led to a dramatic increase in the world record. It involves leaping head-first at the bar and twisting in mid-air, so that the front of the body faces upwards. The technique shifts the athlete's centre of mass to the underside of the body as it passes over the high jump bar, thereby keeping it as low as possible.
Boxes
Box 1. Olympic track and pool facilities
Box 2. Perfect timing: Timing devices and reaction time
Box 4. Rackets and balls
Related sites
Physics, technology and the Olympics (PhysicsWeb, UK)
Tennis racquet physics
(Dr Rod Cross, University of Sydney Physics Department, Australia)
The physics of baseball
(Dr Rod Cross, University of Sydney Physics Department, Australia)
Shoe design 101 (Slam Dunk Science, USA)
Page updated June 2003.