Kinetics
Impulse of Force and Momentum
From Newton's Second Law of Motion indirectly follows also the relation between the duration of force and the change to motion of a human body or any object. This relation can be described with the help of two quantities: impulse of force and momentum. Impulse of force is the product of the resultant force ΣF and the duration of this force Δt, if the force is constant. Impulse of force is the cause of changes to motion and therefore changes to momentum. Mathematically this can be expressed as:
where m is the mass of the moving body, vfinal is the velocity at the end of the time interval in question, vinitial is the velocity at the beginning of the time interval in question.
The expression m(vfinal – vinitial) describes the change to momentum Δp and the expression ΣFΔt describes the impulse of force I.
To change momentum of a body we need to change its velocity or its mass. In sport and physical exercise there are mostly no changes in the mass of a human body, therefore a change to momentum is caused almost exclusively by changes to velocity.
Resultant external force acting on a human body for certain time causes changes to its momentum.
To cause greater change to momentum we have to either use greater force for the same period of time, or the same force for a longer period of time.
How to use impulse of force to increase momentum in sport?
In many sports the elementary motor task is to change as much as possible the velocity of human body, its parts, or of an object. In throwing, kicking, tossing, and jumping the projectiles at the beginning of their motion have zero velocity. When fulfilling a motor task, for example throwing javelin, we are trying to give the javelin at the end of our motion the greatest possible velocity. We are therefore trying to increase the momentum of the javelin. The same follows for certain strokes in tennis, boxing and other sports. The important thing is that to change momentum we have to either use greater force or increase the duration of the same force.
The greater the impulse of force, the greater the change to momentum of a body (projectile, human body, tennis racket, ball, etc.)
When throwing light objects, the technique used (duration of force) is much more important for the longest possible throw than the magnitude of the force. When throwing heavy objects, the magnitude of the force is more important. Shot-putters are usually stronger and bigger than javelin throwers. Their preparation is focused on enhancing their ability to exert great force (ΣF in the impulse of force). Top javelin throwers do not have the same maximum strength as shot-putters but they are successful because their technique maximizes the duration of the force they exert (Δt in the impulse of force).
How to use impulse of force to decrease momentum in sport?
Certain sport activities, on the other hand, necessitate decreasing a high initial velocity of a human body (or an object) to zero velocity. In other words we need to decrease momentum of given bodies. As examples we can easily imagine all kinds of landing, catching of balls, pucks, etc.
For example pole vaulters cannot land on their feet, due to the technique they use, but they land on their backs. Landing on the hard ground means a very short time of deceleration of human body, so the forces acting on human body in such a situation would be massive and devastating. Landing pads are made of soft and floppy material which prolongs the time of breaking the fall. The impulse of force is the same as in landing on the ground but its magnitude is decided rather by the duration of landing than by the magnitude of the force. Gymnasts also do not land on the ground but on specially designed soft and flexible pads that prolong the time of breaking the fall to decrease impact forces acting on gymnasts.
Boxing gloves, catching glove of an ice hockey goal keeper, running shoes, and all kinds of other sporting equipment with the task of reducing substantial forces generated in contact with other bodies are based on the same principles.