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Net force
the sum of all forces acting on a body, also termed resultant force. It is the overall force acting on a body when all individual forces have been considered.
Balanced forces
these occur when two or more forces acting on a body are equal in size and opposite in direction. Net force =O, the body will remain at rest or in motion with constant velocity.
Unbalanced forces
these occur when two forces are unequal in size and opposite in direction. A net force will be present and the body will change its state of motion, either accelerating or decelerating.
Weight
the gravitational pull that the earth exerts on a body →Weight (N) = mass × acceleration due to gravity.
Reaction
the equal and opposite force exerted by a body in response to the action force placed upon it.
Friction
the force that opposes the motion of two surfaces in contact.
Air resistance
the force that opposes the motion through the air. laws of motion, force and the use of technology
Streamlining
the creation of smooth air flow around an aerodynamic shape to minimise air resistance
Free body diagram
a clearly labelled sketch showing all of the forces acting on a body at a particular instant in time.
Reliability
the extent to which an experiment, test or measuring procedure gives the same results after repeated trials
Validity
how well a test measures what it claims to measure. It is important for a test to be valid in order for the results to be accurately applied and interpreted.
Force
a push or pull that alters the state of motion of a body.
Inertia
the resistance of a body to change its state of motion, whether at rest or while moving.
Velocity
the rate of change in displacement. This is a term used throughout this chapter and, although closely related to speed, includes a directional element.
Stability
the ability of a body to resist motion and remain at rest, or for a body to withstand a force applied and return to its original position without damage.
Mechanical advantage
second-class lever systems where the effort arm is greater than the load arm. A large load can be moved with a relatively small effort.
Mechanical disadvantage
third-class lever systems where the load arm is greater than the effort arm. A large effort is required to move a relatively small load.
What is Newton's first law?
aka. 'The Law of Inertia' "A body continues in a state of rest or uniform unless acted upon by an external or unbalanced force."
give an example of newtons first law
For example→ So, a 100m sprinter in the blocks will remain in the blocks and not accelerate until an external force large enough to overcome their inertia creates motion. The greater the mass of the sprinter, the greater the force needed to accelerate away. Equally, once they reach a constant velocity, they will remain at this velocity until acted on by external, or unbalanced forces.
what is newtons second law of motion
aka. 'The Law of Acceleration' "A body's rate of change in momentum is proportional to the size of the force applied and also acts in the same direction." The greater the mass of a body, the greater the force needed to overcome inertia and create motion. The quantity of this motion is called momentum.
give an example of newtons second law
For example →So, a 100m sprinter in the blocks will want to apply as much forward force as possible to accelerate away from the blocks quickly in a forwards direction. The greater the force applied, the greater the rate of change in momentum, and therefore acceleration from the blocks. The force is applied in a forwards direction, meaning the sprinter will drive to the finish line.
what is newtons third law
aka. 'The Law of Reaction' "For every force applied to a body, there is an equal and opposite reaction force." An action force is generated and applied by the athlete to the ground, or an object. A reaction force is the equal and opposite force generated by the ground or the object back to the athlete.
give an example of newtons 3rd law
For example → So, when the 100m sprinter applies a down and backward action force into the blocks, the blocks provide an equal and opposite up and forward reaction force to the sprinter to drive them out the blocks.
what are the 2 types of force
internal and external
what is internal force
generated by the contraction of skeletal muscle. A 100m sprinter must contract the rectus femoris to extend the knee and gastrocnemius to plantar flex the ankle to generate the force required to drive away from the blocks.
what is external force
comes from outside the body and acts upon it. We consider the forces of weight,reaction,friction and air resistance
what are the effects of force
Force can change the shape of a body
force can create motion
Force can accelerate a body
Force can decelerate a body
Force can change the direction of a body
Force can change the shape of a body
if the goalkeeper fails to make the save, the force of the ball coming into contact with the net will make the net change shape.
force can create motion
the football will remain at rest on the penalty spot until a force is applied to make it move.
Force can accelerate a body
the greater the force applied by the footballer's foot to the ball, the greater the rate of acceleration towards the goal.
Force can decelerate a body
as the ball moves through the air towards goal, the force of air resistance will act in the opposite direction and slow it down.
force can change the direction of a body
as the goalkeeper dives to save a high corner shot , he will apply a force from his hands to the ball changing its direction and pushing it way from the goal
what are the 2 vertical forces
weight and reaction
describe weight as a force
Gravitational pull (10m/s/s) exerted through the body's COM towards earth. Weight (N) = mass (kg) x gravitational acceleration (m/s/s) Arrow from COM vertically down to earth
describe reaction force
The equal and opposite force exerted in response to the action force (N). If the reaction is equal to weight then there is no upwards movement. if R > w then there is upward movement Arrow from point of contact vertically upwards.
What are the 2 horizontal forces?
friction and air resistance
describe friction
Opposes the motion of two surfaces Arrow from point of contact in the direction of travel.
factors affecting friction
The roughness of the surface; The roughness of the contact surface; Temperature; Size of normal reaction.
describe air resistance
Opposes motion of a body travelling through air. Arrow from the Centre of mass in the direction of travel.
factors affecting air resistance
Greater velocity = greater resistance Shape affecting the flow of air around an object (overcome by streamlining) Frontal cross-section
key points for free body diagrams
Big Label each force arrow and its force R>w unbalanced forces, positive net force and vertical acceleration) Always draw a direction of motion
Arrows on a free body diagram represent
Weight
weight free body
always down and from the centre of mass
reaction free body
always up and from point of contact with the floor
air resistance free body
→ opposite direction to motion and from centre of mass
Friction free body
usually in the direction of motion and from contact with floor
calculating force
Force (N) = mass (kg) x acceleration (m/s/s)
calculating momentum
Momentum (kgm/s) = mass (kg) x velocity (m/s)
calculating acceleration
Acceleration (m/s/s) = (final velocity
calculating velocity
Velocity (m/s) = displacement (m) / time taken (s)
Calculating weight
Weight (N) = mass (kg) x acceleration due to gravity (m/s/s)
Define centre of mass
the point at which a body is balanced in all directions, the point from which weight appears to act.
describe centre of mass in sport
If in the anatomical position, the CoM will be around their belly button. The way the body is positioned affects the position of the CoM. For example, if an athlete raises their arms, the CoM raises if they bend their knees, then the CoM will lower. It can also move outside of the body to be rotated around. For example, during a somersault.
effect of centre of mass during a fosbury flop
Uses a j curve to allow greater velocity in the approach Plants the outside foot to allow the inside leg to lift along with the arms, at take off to raise the centre of mass as high as possible Fully extends the spine to rotate around the bar moving the centre of mass outside of the body and below the bar Only one section of the body has to be above the bar at any one time Overall , the centre of mass passes underneath the bar; the flop requires less force at take off to clear the same heights. Greater heights can be achieved with this technique
Define stability
→ the ability of a body to resist motions and remain at rest. It is also the ability of a body to withstand a force applied and return to its original position without damage.
maximising stability
Low COM
Minimising stability
Raise COM Reduce point of contact Lean forward to shift line of gravity to edge or out of base of support Small base of support Minimal points of contact Line of gravity falls in front of base of support
factors affecting stability
Mass of the body Base of support Height of the CoM
factors affecting stability ~ Mass of the body
→ the greater the mass, the greater the inertia, so the greater the stability. Do this in order to withstand high forces, like a rugby prop, or sumo wrestler.
factors affecting stability ~ Base of support
→ the greater the size of the base of support, the greater the stability. For example, in a bridge in gymnastics, having 4 contacts. Or, standing on two feet, more than shoulder width apart increases the size of the base of support.
factors affecting stability ~ Height of the CoM
→ the lower the CoM, the higher the stability. For example, when a gymnast jumps and lands they flex the hip and the knee to lower their CoM, also done in martial arts.
2 main functions of lever
To generate a muscular effort to overcome a given load To increase the speed of a given movement
The components of a lever system are:
lever (bone) fulcrum (joint) effort (muscular force) load (weight or resistance) Effort arm → distance between the effort and the fulcrum Load arm → distance between the load and the fulcrum
Effort arm
→ distance between the effort and the fulcrum
Load arm
→ distance between the load and the fulcrum
most common lever in the body
3rd class lever
1st class lever
Middle component ~ Fulcrum Example ~ The neck when preparing to head a football F- neck joint L- cranium weight E- muscle contraction from trapezius
2nd class lever
Middle component ~ load Example The take-off phase of a high jump F- a ball of the foot L- body weight E- gastrocnemius
3rd class lever
Middle component ~ effort Example Bicep curl at the elbow F- elbow L- dumbbell E- force supplied by the bicep brachii
Which lever operates at a mechanical advantage?
2nd class
which lever operates at a mechanical disadvantage?
3rd class
3 technologies
limb kinematics wind tunnel force plates
Limb kinematics
study of movement in relation to time and space. 3D or optical motion analysis records an athlete performing a sporting action or a patient performing normal bodily movement. This allows joint and limb efficiency to be evaluated with measurements of bone geometry, displacement, velocity and acceleration in multiple planes of movement. Computer software is linked to multiple video or infra-red cameras which record, capture and convert the motion shown by the reflective markers placed on the body's joints and bony landmarks into digital format. immediate, objective and highly accurate
limb kinematics advantages
can be used by coaches to adjust technique and improve performance. The motion can focus on a specific limb or piece of equipment and analyse technique in preparation, execution and recovery phases of motion for example, a golf swing or football strike.
limb kinematics disadvantages
However, the accuracy and repeatability of results depends on the correct placement of the bodily markers and the mathematical principles the results are based upon do not cater for individual differences. This technique is highly specialised, expensive and largely limited to laboratory conditions, making some sporting actions difficult.
wind tunnels
objects as small as a cycle helmet or as large as an F1 car may be tested for aerodynamic efficiency. The object is placed inside the wind tunnel with instruments to measure the forces produced by the air against its surface. Engineers may also study the flow of air around the object by injecting smoke or dye into the tunnel.
Wind Tunnel Advantages
The aim of using a wind tunnel is to improve the flow of air around an object, streamlining its path through the oncoming air and potentially increasing lift or decreasing drag. The use of wind tunnels allows engineers to have tight control on environmental variables such as wind speed or wind direction, and gives them the ability to control cross winds and measure air resistance and flow with precision accuracy in a very time-efficient manner.
wind tunnel disadvantages
However, these are very specialised facilities mainly housed in engineering bases. They are very expensive and require complex analysis of the results by research professionals.
force plates
Ground reaction forces can be measured in laboratory conditions using force plates. Data from an athlete balancing, running or jumping on a force plate can be used to assess: the size and direction of forces acting on the athlete, acceleration rates, work power output.
Most commonly, force plates are used for sports biomechanics assessment, gait analysis analysis of human motion mainly for running and posture analysis), balance, rehabilitation and physical therapy.
A force plate is a rectangular metal plate with built-in force transducers usually sunk into the ground to become part of the floor.
When an object or limb makes contact with the force plate, an electrical output proportional to the force being applied is displayed in graphical form on a computer.
The size of the force and time the force is applied can be displayed in three planes of motion.
force plates advantages
immediate, accurate and reliable results
force plates disadvantages
specialist, expensive & usually housed in labs
