Forces in athletics: Newton’s Laws

Force is the effect one body has upon another. Motion in athletics can be produced only when force has been applied, but it is nevertheless possible to have force without motion. In track and field athletics the main sources of force are (a) internal, the muscular actions of the athlete, and (b) external, the downward pull of gravity, the friction and upthrust of the ground and the resistance of the air. As Sir Isaac Newton’s Laws of Motion are fundamental to the whole science of force—even today’s rocket and missile discoveries are based upon his theories—they will be discussed first.

Newton’s First Law (Law of Inertia)

It was as long ago as 1687 that Newton enunciated the first of his three laws in the now famous words: ‘Every body continues in its state of rest, or of uniform motion in a straight line, except in so far as it may be compelled by impressed forces to change that state.’

This is known as the Law of Inertia, inertia (the Latin for idleness or laziness) being the property innate in all bodies, animate and inanimate, by virtue of which they obey this law.

Expressed in simpler language, it says that everything in the universe is lazy, so lazy that force is necessary to get it on the move, when it then travels in a straight line; so lazy that, once in motion, further force must be applied to slow it down, stop it, speed it up or change its direction.

Thus, a shot rolling along level ground would continue to roll for ever in a straight line were it not for ground and air friction. An athlete moving horizontally through space would continue to travel for ever in a straight path were it not for air resistance and the downward pull of gravity. The law applies also to spinning motion. A hammer thrower would go on and on turning were it not for friction and the forces subsequently applied in releasing the missile.

Direct proof of the correctness of Newton’s assumption is impossible here on earth, since we cannot remove a body completely from external influences, but in its logical results the law is never at variance with experience.

Inertia, therefore, is concerned with a body’s resistance to change in movement and is an important—often determining—factor in problems involving energy-expenditure and fatigue in exercise. It is a property always present in an object and is never ‘overcome’ (in the sense that it is destroyed) once that object has been set in motion, as is frequently supposed.

The claim ‘to get an object on the move one must overcome its inertia’ is based on the common experience whereby it is more difficult to start an object moving than to keep it in motion; but this is due to the fact that, to start it, force must be used against both its inertia and the frictional ground and air forces, whereas to maintain a constant speed only the frictional forces oppose movement, and less force is therefore required. But to increase the speed again, inertia has still to be ‘overcome’. For example, ignoring ground and air friction, when a definite force is applied horizontally, the pressure between the hand and a 16-lb shot is exactly the same, whether the shot is moved from rest or is in motion at the time. In terms of inertia, therefore, rest and uniform straight-line motion are manifestations of the same thing.

Inertia is proportional to mass—the amount of the material of which an object is made. In fact, the terms are often interchangeable. The mass of an object is the measure of its inertia, its resistance to change in motion. The British unit of mass is the pound (lb). By way of illustration, if equal forces are applied to a 16-lb shot and a Mb 12-oz javelin, the change in motion of the javelin (i.e. its acceleration) will be much greater. Mass must not be confused with weight; we shall see later that they are not the same.

Newton’s Second Law (Law of Acceleration)

By means of the second law, which actually includes the first, we can determine how force can change the motion of an object. It states: ‘The rate of change of momentum is proportional to the impressed force, and the actual change takes place in the direction in which the force acts.’

Momentum is the product of mass and velocity: it can be considered as a measure of the quantity of motion possessed by a body. It is a vector quantity, i.e. it possesses both magnitude and direction. A runner having a mass of 140 lb and a velocity of 30 ft per second has a momentum of 4,200 units, and so has a 30-lb object moving at 140 ft per second. Thus, theoretically, a marble might be made to produce as much havoc as a cannon ball, if it could be given sufficient velocity.

At a definite time the moving runner has momentum, and a period of time is necessary for him to change it. From the law of inertia we know that the velocity of a moving object (i.e. the distance it travels per unit of time in a given direction) remains constant unless a force acts on it. This second law tells us that, provided we consider a definite period of time, any change in velocity (positive or negative) will be directly proportional to the amount of force used; it will also be inversely proportional to the object’s mass.

For example, consider a definite period of time, say, two seconds; the effective force a runner must use to increase his speed during these two seconds in a race is in direct proportion to his speed increase, i.e. if he wishes to double this speed increase in the two seconds he must double the amount of force he exerts.

If there are two runners, one weighing only half as much as the other, and the same effective horizontal force is exerted by each during the two seconds, the lighter athlete will acquire double the speed increase of the heavier one—demonstrating that the change in velocity is inversely proportional to the mass of the object. This second law therefore generalises the obvious fact that one can throw a cricket ball farther and faster than a 16-lb shot.

It should be noted, however, that a force need not act through an object’s Centre of Gravity to produce a given linear acceleration, for the force will create the same linear acceleration whether or not so directed. Should the force not pass through the Centre of Gravity, that point will change speed in a direction parallel to the direction of the force while, simultaneously, the object will rotate about an axis passing through its Centre of Gravity.

Newton’s Third Law (Law of Reaction)

For every force acting anywhere there is always an equal force acting in an opposite direction. Forces always work in pairs, as ‘twins’ opposing each other. This is the meaning of Newton’s Third Law, which states: ‘To every action there is an equal and opposite reaction; or the mutual actions of two bodies in contact are always equal and opposite in direction.’

The effect upon one body is known as the action and that upon the other the reaction, but it is often a matter of choice or opinion as to which is which.

For example, if two spring balances are hooked together and then pulled in opposite directions the readings on them will be identical. A man standing on the ground is pulled down on it with his weight and the ground pushes up with a force (often referred to as ground reaction) equal to his weight. When a bird beats downward with its wings in flight, its body and the air struck experience equal but opposite forces. And when a sprinter leaves his blocks he drives backward with a force equal to that which propels him forward.

For as long as a runner or jumper accelerates his swinging leg upward at take-off, the tendency will be for the rest of his body to be pulled down; hence he could be considered effectively heavier at that instant. If the leg ceases to accelerate, but continues upward with a uniform motion, then his take-off foot presses against the ground with a force equal to his weight; and when it slows down momentarily he loses weight again. In a reverse movement, with the leg dropping (but with his other foot still in contact with the ground) the pressure between his supporting foot and the ground decreases and then increases. These examples of action and reaction and the two-way action of forces can be demonstrated on a weighing machine or, more scientifically, on a force platform.

When a force sets an object in motion the momentum of that object is altered. As, according to this Third Law, forces always work in opposing pairs, a change in momentum in one object must be accompanied by the same change in momentum of another object in an opposite direction and the momentum of the system as a whole therefore remains the same. Hence, when a gun is fired, the explosion gives the bullet momentum in a forward direction equal but opposite to the gun’s ‘kick’ against the shoulder.

Again, a high-jumper’s momentum at take-off is equal to the change in the earth’s momentum in an opposite direction. But the reaction of the earth is not noticeable because of its enormous mass. Likewise, such is the law of reaction, the earth moves up to meet the athlete as he falls towards the earth. The earth’s mass is so great in comparison that both movements can be neglected.

The principle holds good, too, for mid-air movements of parts of an athlete’s body. Although, in the air, he can do nothing to change the momentum of his body as a whole nevertheless he can move a part of it against some other part, producing similar reactions within the body.

It should be noted, however, that to exert a maximum of body force in running, jumping or throwing the athlete must be in contact with firm, resisting ground. If, for example, in shot putting he breaks contact before completing the delivery, then he can only impart additional speed to the shot by giving his body backward speed in reaction. In this case his final arm thrust is made at the cost of reducing the shoulder’s forward speed; thus, the force and range of arm movement are reduced.

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