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Matthew Brown ND1C


Task 2

(P2) Define the terms to describe mechanical motion, including reaction forces, velocity, acceleration, centre of mass, centre of gravity and angular velocity.

(M2) Apply these terms to examples involving sporting motion.

(D2) Carry out suitable calculations and use these terms to analyse sporting motion.

In this task a will be explaining the mechanical principles. The first thing I will do is define most of the terms used in biomechanics. I have got an example of a 400m run that has been broken down into 50m splits. I will use this example to relate my explanation to sport.

Subject A





















Fig 1.1

Speed    =                 distance                m/s


Velocity   =                 displacement                m/s


Acceleration =        final velocity                m/s2


Above is the equation for speed, velocity and acceleration.


Internal – an internal force is a force that acts with in a system. Like contracting muscles in the body.

External – an external force is a force that acts on an object from the environment. Like the wind blowing some one over or friction grinding a car to a stop.

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(Fig 1.2)



Equilibrium – equilibrium is a balance of forces. A body is said to be at equilibrium if it is still or moving with uniform velocity. In a 400m run if there is one point where all forces balance out then this is equilibrium.

Magnitude – this is basically size, it can be measured in different units. In most sports the unit is metres like the 400m example completed by subject A.

 “The property of relative size or extent (whether large or small)”


Linear motion: is when all points on a body move the same distance, in the same direction, and at the same time. 

Displacement – this is a vector quantity that refers to how far an object has moved. In sport if an athlete completed a 400m race they would have a displacement of 0m.

Vectors – are quantities described by both magnitude and direction. If athlete ran 100m north this is a vector measurement.

Scalars – are quantities described only by size or magnitude. If an athlete ran 100m this is a scalar measurement.

Velocity – is a vector quantity that refers to the rate at which an object changes in position.

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If torque acts on a spinning object it will change its angular velocity. The object can speed up or slow down. In dancing the partner will spin the partner just below the wrist so they have a better lever as the lever arm is longer then if the spun them by the shoulder.

Conservation of momentum - It is well known that the momentum of an object will stay the same provided no external force acts on it. This works the same way with angular momentum, a spinning body will remain the same (provided no external forces act).

A body which is spinning will keep its value of angular momentum once the movement has started. Therefore if MI (moment of inertia) (I) changes by changing body shape then angular velocity must also change to keep angular momentum (H) the same. If MI (I) increases (body spread out more) then angular velocity must decrease (rate of spin gets less). Like an ice skater when there arms are spread out wide they have a higher moment of inertia which slows down the rate at which they spin. When they lift there arms above there head they lower there moment of inertia which increases there rate of spin.

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