Module 2562 A.2.1

FORCEFORCE• FORCE is push or pull

• the unit is the NEWTON (10 N is approx the weight of 1 kg)

• force changes the state of motion of an object• force causes acceleration or deceleration or change of direction• the more force the bigger the acceleration

• force changes the shape of an object

• WEIGHT• FRICTION• REACTION FORCES• AIR RESISTANCE / FLUID FRICTION

– all these forces affect the sportsperson

Motion and Movement - Newton’s Laws

Module 2562 A.2.2

NEWTON’S FIRST LAW of MOTIONNEWTON’S FIRST LAW• this law is used when ZERO NET FORCE is

applied to an object• this doesn’t mean that zero force acts, but

that all forces MUST CANCEL OUT

• with zero net force an object– is STATIONARY or– moves at CONSTANT SPEED in the

SAME DIRECTION

• a sprinter in full stride has four forces acting• but they cancel out exactly• therefore he / she travels at constant speed

Motion and Movement - Newton’s Laws

Module 2562 A.2.3

NEWTON’S SECOND LAW of MOTIONNEWTON’S SECOND LAW

• this law is used when a NET FORCE acts on an object

• net force FORWARDS produces ACCELERATION

• net force BACKWARDS produces DECELERATION

• net force SIDEWAYS produces CHANGE OF DIRECTION

• the bigger the force the bigger the acceleration

• the sprinter slows down at the end of a race

• there is a net force backwards• so the sprinter decelerates

Motion and Movement - Newton’s Laws

Module 2562 A.2.4

NEWTON’S THIRD LAW of MOTIONNEWTON’S THIRD LAW

• this law is used when two bodies EXERT FORCES ON ONE ANOTHER

• ACTION AND REACTION ARE EQUAL and OPPOSITE IN DIRECTION

• action of jumper down on ground= reaction of ground up on jumper

• the harder you push down on the ground, the more the ground pushes up on you

• this upward force on the jumper is the force acting to cause the take off

Motion and Movement - Newton’s Laws

Module 2562 A.2.5

The EFFECTS of FORCEEFFECTS of FORCE• force causes linear acceleration or deceleration• including change of direction

• the point of action of a force affects what happens• friction acts at the feet of a sportsperson, not enough of it

and the person’s feet slip

• if a force acts through the person’s centre of mass (CofM), then linear motion is caused

• if a force acts to one side of the CoM then rotation is caused

• like take-off in the high jump, the reaction force acts to one side of the CoM

Motion and Movement - The Effects of Force

Module 2562 A.2.6

CENTRE OF MASSCENTRE of MASS (CoM)• this is the single point in a body which represents all the

spread out mass of a body

• the weight acts at the CoM since gravity acts on mass to produce weight

Motion and Movement - Centre of Mass

Module 2562 A.2.7

CENTRE OF MASSWHERE IS THE CENTRE OF MASS?• position of centre of mass depends on

shape of body

• this is how the high jumper can have his CoM pass under the bar

• but he could still clear the bar

Motion and Movement - Centre of Mass

BALANCE• to keep on balance

the CoM must be over the base of support

Module 2562 A.2.8

INTERNAL FORCES and LEVERS

LEVERS• a lever is a means of applying force at a distance from the

source of the force

Motion and Movement - Levers

INTERNAL FORCES• this describes the forces acting when

a muscle pulls on its origin and insertion

• the force on the origin H is equal in size but opposite in direction to the force on the insertion U

• this changes the shape of the person

• a lever has a FULCRUM (pivot), EFFORT and LOAD

• in the human body, usually a joint and the attached limbs or bones act as a lever

• FORCE is applied as effort by a muscle or group of muscles

• the LOAD is the force applied to the surroundings by the lever

Module 2562 A.2.9

LEVERSEFFECTS OF THE LENGTH OF LEVER• the length of the lever (d in the diagram) affects the

load able to be exerted by the lever, and the speed at which the hand can move

• the longer the lever d, the smaller the value of the load for a given biceps strength

• the longer the lever arm d, the faster the load can be applied (as the limb moves through its range - a longer limb - the hand - would move further in the same time)

EFFECTS OF THE ANGLE BETWEEN EFFORT AND LEVER ARM• the effect of the effort is affected by the angle between

the direction of the effort and the lever arm• optimum load is exerted when this angle is 90o

Motion and Movement - Levers

Module 2562 A.2.10

EXAMPLES OF LEVERS

Motion and Movement - Levers

TRICEPS ELBOW LEVER• class 1 lever : pivot between effort and

load• see-saw lever found rarely in the body

CALF MUSCLES / ANKLE LEVER• class 2 lever : load between pivot and

effort• wheelbarrow lever, load bigger than effort

QUADS KNEE LEVER• class 3 lever : effort between pivot and

load• mechanical disadvantage, effort bigger than

load, most common system found in body• another example : biceps / elbow