Newton's Laws

To produce a sporting movement our bodies must deal with and utilise a number of forces that are working in different and often conflicting ways. Elite performers are able to manipulate these forces for their advantage and the understanding of the physics behind movement is of prime interest to sports scientists and sports coaches. Newton’s three laws are used by sports scientists to explain how the performer is able to control their movements in precise and not so precise ways.

‘A body will remain at rest or in constant uniform motion unless a force acts upon it’

Newton’s first law has two components. Firstly, an object that is stationary will not move unless a force is applied to it. Secondly an object that is moving will travel in the same direction and at the same velocity unless a force acts on it.

There are a few considerations that need to be understood when applying this law to sport. We can see this when looking at some of the shots a golfer will play. When a golfer places his/her ball on a tee at the first hole, the ball will remain (at rest) on the tee until the golfer swings their club to hit the ball (applies a force). If the ball was hit in space (where there is no gravity) the ball would continue to travel at the same velocity and in the same direction, forever … certainly until a meteor, satellite or planet got in the way! On earth, the ball would travel but air resistance and gravity (a force!) would act on the ball, preventing the ball from travelling in uniform motion. Indeed if you are a bad golfer then a tree, bush, fence or other hazard might prevent the ball flying in constant uniform motion! If a ball has been hit high over a tree it will slow down at the top of the flight but will speed up as it is travelling back to the green.

‘The rate of change in momentum (or acceleration) of an object is directly proportional to the force applied to it and in the same direction as its application’.

Newton’s 2nd (law of acceleration) states that the rate at which a body or object changes acceleration depends on the size of force being applied. This force also gives the direction that the object will move. The more force applied the more the acceleration is achieved which means the greater the weight an athlete can lift or the more vertical acceleration achieved on take off for a high jump.

If we look at a tennis player playing a number of successful smash and volley shots at the net we would be able to see Newton’s second law in action. It is doubtful that any of these shots would be identical. When elite players hit a smash shot they may hit the ball with so much force that it bounces in their opponents’ side of the court then bounces over their head. However, a volley will be delicately angled into the corner of their opponent’s court travelling with less velocity so it is impossible for them to reach it before it has bounced twice. The ball’s acceleration will depend on the amount of force the player hits the ball with, so to hit it with a greater acceleration the player will swing the racket faster. To alter the direction of the ball, the player will make contact with it so it travels in the desired direction; hitting the ball closer to its top means it will bounce close to the net.

‘For every action there is an equal and opposite reaction’.

Newton’s 3rd law (of reaction) states that for every force there is an equal and opposite reaction force, meaning that as a diver pushes into the diving platform, the board will exert an equal and opposite force back to the diver.

When we look at the results of a vertical jump or Sergeant jump test we see that elite sprinters and high jumpers can jump over 80 cm into the air, whereas the average A level PE student will be lucky to reach over 40cm. To explain the differences in the results we need to apply Newton’s second and third laws. We have seen how the acceleration of an object relates to the amount of force applied by the leg muscles (Newton’s second law). Newton’s third law is also a factor because when the muscles contract they transmit the force to the ground (known as ground reaction forces [GRF]). As the ground has a greater inertia than the jumper it does not move. However, an equal and opposite force is applied from the ground to the person so that they travel into the air (vertical component). The greater the force applied the greater the force applied back by the ground (Newton’s third law) meaning the faster the jumper will accelerate the further they will travel (Newton’s second law). When performing a standing long jump the same principles apply but this time the force that the jumper applies is in a different direction so they travel forwards as well as upwards. A person will lift off the ground if the force applied to the ground is greater than gravitational forces. Thus, in the vertical jump example, a person’s ability to apply forces to the ground, through the forces applied through muscular contraction, will affect how high they jump.