Chapter 3:Linear Motion

Chapter 3:Linear Motion

1. Linear= straight line

   • Involves movement in a single direction e.g. speed, acceleration + force

2. Scalar= a quantity which has size or value only

   • Example= mass, speed, energy, length
   • No directional angled need when describing these quantities
   • Speed= distance travelled per second
     • Measured in metres per second
     • Speed= distance travelled (m) divided by time take to travel (seconds)

3. Vector

   • Velocity
     • Velocity= distance travelled (metres) divided by time take to travel (seconds)
       • In a given direction
   • Quantity that has magnitude + direction
     • Example= weight, velocity acceleration
   • Force
     • Forces going in opposite directions can cancel out to give a 0 net (resultant) force
       • Newton's 1st law occurs= object will be stationary or constant velocity
       • Situation called equilibrium, When stationary= static equilibrium , when at constant velocity= dynamic equilibrium
     • Resultant force= 2 or more vectors added together, taking into account their directions
       • Idea that net force causes acceleration linked to Newton's 1st and 2nd laws of motion
       • If forces don't cancel out = unbalanced resultant and the net force acts, causing acceleration of body in direction of the net force
     • Vertical forces= weight, ground reaction force
     • Horizontal forces= friction force , drag
   • Are mathematical rules that enable you to add together vectors not in the same direction. Draw from forces a line so meet at right angle (makin a quadrilateral shape) the resultant force lies along the diagonal of the shae
     • Possible to calculate size + direction of resultant vectors via trigonometry
       • See page 57 for example
       • If force passes through centre of mass no toppling or rotation occurs

4. Graphs

   • Direction shown by angle 0 (measured in an anticlockwise direction to the X axis- of an arrow drawn on a graph
   • Magnitude represented by length of arrow on graph
   • Distance/ time graphs
     • Shows progress of an object as it moves along
       • Go to page 59 for examples
     • Straight line on graph= no movement I,e, the distance remains the same as time goes along
     • Line curving upwards= distance changes with time= object moving forward
       • Line levelling off= slows down than stops moving (speed reduces to 0)
     • What changes is the gradient= velocity= distance divided by time
       • Gradient= as the rate of change of speed with time
         • Indicates speed= distance divided by time
         • Steeper the gradient of the graph the faster the movement= the greater the speed or velocity
   • Seped/velocity against time graphs
     • shows how velocity changes with time
     • Gradient of line= acceleration of object
       • Acceleration = change of velocity divided by time taken
       • Straight line= no acceleration
       • Diagonal line going upwards=velocity changes continuously with time= constant acceleration
       • Curve line upwards= acceleration differs
       • See page 60/61 for visual examples

5. Speed +velocity

   • involves idea that a body or object changing its position
   • distance measured from 1 point to another point + tells use where athlete are in a space (pitch or court\0
     • Important for races where starts + finishes fixed
     • Displacement
       • is the distance between the start position and the position of the athlete (direction is usually taken into account)

6. Acceleration

   • Is the change of velocity per second
   • Acceleration= final velocity- starting velocity divided by time taken to change
   • Acceleration also if velocity remains the same but you are changing direction , for example an object following a circular path
     • Like the head of a hammer in a hammer throw= chance in direction=change in velocity = acceleration towards centre of circular path + force occurs towards centre of circular path

7. Weight + mass

   • Mass= scalar= represents the total quantity of matter in an object
     • linked closely to idea of inertia= resistance to acceleration
       • More inertia an object has the harder it is to accelerate when a force is applied
         • Why athletes who need speed/ change direction rapidly or accelerate from rest aim to have the least body mass possible consistent with the necessary strength
         • equation F= M x A
         • Inertia is a property of mass (therefore all objects)
         • Applies to decelerating objects , so once moving an object requires force to slow it down/stop it
           • Example= runner have difficult stopping after sprinting due to inertia of their mass
   • Weight= force due to gravity on mass(with a direction towards the centre of the earth)
     • Gravitational field strength changes slightly depending on thickness of earths crust, the longitude, proximity of large mountains + height above sea level
     • 9.81 newtons for each kg of mass + acts on the centre of mass of a body
     • Is predominant fore acting on an object projected into flight
     • Force field
       • Example = gravity, electromagnetic, nuclear fields
       • is a means by which a force csn be exerted without touching - e.g. ball in flight accelerated towards earth by gravity so has net force acting on it without being in contact with earth
         • Experiment done in a vacuum to confirm concept of non-touching force

8. Friction

   • Force that acts sideways between 2 surfaces which tend to slide pass 1 another
   • Enables athletes to slow down, swerve, walk + run
   • Magnitude of friction dependent on grip of footwear on floor surface + surface itself
     • Studs + spikes= increase friction to enable better swerving + acceleration/deceleration in games/track on soft/wet surfaces
     • Solid smooth rubber soles give better friction on dry hard surfaces - used in rock climbing shoes, tennis shoes for concrete
     • Long slender footwear (skis or skates) worn in snow + ice as have low forward friction but high sideways friction
   • Friction acts forwards on feet of acceleration runner
   • Depends on the force pressing the surface together (not the area of contact)
     • Friction enables swerving by players- force acts sideways to the direction of motion changing the direction of motion
       • Direction ball takes after bounce depends on direction of spin + friction between ball and ground
   • Rolling friction
     • Describes the force between surfaces which don't move relative to one another e.g. wheel rolling over a surface
     • Friction can be anything from 0- to a maximum (just before slipping occurs) . Post slipping the friction force falls, so not enough friction is present to keep a person from falling over
   • Sliding friction
     • occurs when 2 surfaces are moving relative to 1 another.
     • Is always less than maximum rolling friction = why advance braking systems reduce braking force on wheels if sensor detect beginning of sliding
   • Fluid friction
     • (drag) is a term applying to object moving through fluids (gasses or liquids). Force acts in opposite direction to motion direction
       • Hence air resistance occurs be object moving through air

9. Muscle contractions

   • Action and reaction force occur in body when muscle contracts
     • Involves 2 end of the muscle (insertion and origin) pulling equally on 1 another in opposite directions
     • Effect on body shape/relative position of limbs + attatchments depends on which end of muscle can move
   • Look at page 62 for examples of muscle contraction causing changes in body shape due to origins + insertion pulling towards one another
     • Examples= tumbling in gymnastics

10. Reaction forces

    • Newton's 3rd law= explains what happens with 2 bodies exert forces on 1 another. Action + reaction are equal and opposite and always occur in pairs
    • Action= acts on one of the bodies
    • Reaction= from action a reaction acts on the other body
    • Example= sprint start athlete pushes on block (action) and block push's forward on athlete (reaction force)= acceleration of athlete
    • More examples on page 63