P4- EXPLAINING MOTION

P4- EXPLAINING MOTION

1. SPEED-

   • Speed is measure in metres p/second (m/s) or kilometres p/hour (km/h)
     • speed= distance (m)/ time (s)
       • measuring speed over a short period of time will give you INSTANTANEOUS SPEED
     • most things don't travel at a constant speed so we calculate average speed

2. DISPLACEMENT

   • the displacement of an object is the DISTANCE from its start point in a straight line
     • e.g. when you run aound a 400m track you've run 400m, but you're displacement is zero because you are back at the start again.
       • displacement is a vector quality
         • it has a size and a direction
           • VELOCITY= speed in a certain direction
             • velocity is also a vector
             • instantaneous velocity= speed and direction at any given time
               • negative velocity is when the vehicle is travelling in the opposite diection
           • AVERAGE VELOCITY= DISPLACEMENT (m)/ TIME TAKEN (s)

3. DISTANCE TIME GRAPHS

   • time plotted on x axis(horizontal)
     • distance plotted on the y axis (vertical)
       • (upright) STRAIGHT LINE= CONSTANT SPEED
         • FLAT LINE= STATIONARY VEHICLE
           • CURVED LINE= CHANGING SPEED
   • GRADIENT=SPEED
     • steeper the gradient= faster the speed
     • gradient= change in y/change in x

4. DISPLACEMENT TIME GRAPHS

   • negative and positive axis
     • going and coming back route
     • when the vehicle returns to its starting point the displacement= zero
     • displacement is negative if the vehicle is travelling behind its starting point
   • flat line=stationary
     • gradient=speed

5. ACCELERATION

   • is the rate at which an objects speed increases
     • measured in metres p/second squared
       • ACCELERATION (m/s2)= CHANGE IN VELOCITY (m/s)/ TIMe TAKEN (s)
       • when the net overall force is zero, acceleration is zero.
         • deceleration is when an objects speed slows down, this will be because a force is acting on it, causing it to slow down or decelerate
     • rate in which velocity is changed
       • calculate change in speed as final speed- initial speed

6. SPEED-TIME GRAPHS

   • shows changes in speed during a journey
     • speed = y axis
       • time= x axis
   • similar to velocity time graphs except they don't show direction
     • gradient=acceleration

7. VELOCITY TIME GRAPHS

   • steeper gradient= faster acceleration or decleration
     • HORIZONTAL LINE= CONSTANT SPEED
     • GRADIENT=ACCELERTAION
     • uphill= acceleration downhill=decleration
     • curve= changing acceleration
   • negative velocity= object travelling in opposite direction
   • stationary when velocity is at 0
   • positive velocity= one direction negative velocity=opposite direction

8. FORCES

   • a force is the push or pull that acts between two objects
     • they ALWAYS act in pairs
       • e.g you push against a wall the wall pushes back against you
       • an ATTRACTIVE force pulls objects towards each other
         • a REPULSIVE force pushes objects apart
           • the SIZE of the force is ALWAYS the same on both objects
             • a force always acts in EQUAL and OPPOSITE directions
               • e.g. a ship floats on water, the upthrust (buoyancy) pushes u on the ship and the ship pushes down on the water. This is a repulsive force pair.
                 • e.g. when you stand on the floor, the force is equal to your weight. The floor exerts an upwards force on you, called the REACTION FORCE This reaction force is equal and opposite to your weight.
                   • if you jump upwards you need to push harder on the floor- the reaction force then increases and you accelerate upwards.
                   • forces are vector qualities so they have a size and a direction
                     • a force is represented by an arrow, the direction of the arrow shows the direction of the force and the size shows the magnitude of the force.

9. FRICTION

   • is a force which acts between two surfaces. as the two surfaces slide over each other friction acts to oppose the motion.
     • the size of friction force depends on: the roughness of the surface (more rough=more friction) and how hard the forces are pushed together (heavier the object=more friction)
       • when you try to push an object along a surface, friction will be WQUAL to the APPLIED force and it will act in the OPPOSITE direction so the object will NOT move.
         • as you increase the applied force friction will also increase.
           • eventually the friction reaches its maximum value and the object will move- this is called LIMITING FRICTION
             • the KINETIC ENERGY of the moving object is transferred to eat energy in both surfaces.
               • LUBRICATION (oil) is used to reduce friction between moving parts of machinery to stop them getting too hot and wearing out.
         • you need fiction in order to walk- when you walk your feet PUSH against the friction on the floor, so the floor pushes your foot forwards.
           • the RESULTANT FORCE acting between your foot and the floor is a combination of FRICTION and the reaction of the SURFACE. These forces combine to push the foot in a diagonal upwards direction.

10. THE EFFECTS OF FORCES

    • there are usually several forces acting on an object at one time. The RESULTANT FORCE is the total/overall force acting on an object.
      • when you add the forces you must account the size and direction of the force
        • When Resultant Force= 0 the forces are BALANCED and acceleration= 0. The object will move at a constant seed in a straight line.
          • e.g. in space there is no friction so once an object starts moving it should keep moving at the same speed.
          • RESULTANT FORCE
        • when the forces are unbalanced there is a net force on the object and it will change direction, speed up or slow down.
    • TERMINAL VELOCITY
      • objects falling towards the ground accelerate due to gravity. The force of gravity always acts towards the centre of the Earth.
        • the atmosphere creates an upwards force that slows down falling objects this is known as AIR RESISTANCE or DRAG
          • drag acts in the opposite direction to the speed or velocity of an object.
            • drag forces increase as the velocity of an object increases.
              • the larger the SURFACE AREA= the larger the DRAG FORCE
                • the constant maximum speed reached by a falling object is known as its TERMINAL VELOCITY.
                  • when a person is skydiving- at first the force of gravity is larger than the drag force so the skydiver accelerates. AS the speed increases so does drag, so acceleration decreases. When drag equals the force due to gravity there is no resultant force and the acceleration is zero. The object continues at terminal velocity.
                    • equal and opposite directions.
    • COLLISIONS AND MOMENTUM
      • large forces are involved in collisions. The size of the force depends on the MASS of the object (heavier=larger force) the SPEED of the object (faster=larger force) and the DURATION of the impact (longer the time to stop=lower force)
        • as a result cars and other vehicles are fitted out with safety devices that are designed to INREASE IMPACT TIME, reducing the force of the collision.
          • CRUMPLE ZONES- crumple on impact, AIR BAGS-slow you down, SEAT BELTS- stretch slightly, increasing time for wearer to stop and reducing forces on chest, CRASH HELMETS provide padding that increases time taken for head to hit something hard.
            • MOMENTUM- a resultant force will change an objects momentum. larger force exerted=larger change in momentum
              • MOMENTUM (kg m/s)= MASS (kg) x VELOCITY (m/s)
                • CHNAGE IN MOMENTUM= RESULTANT FORCE (N) X DURATION (S)
                  • force is equal to the rate of change in momentum FORCE (N)=change in momentum/duration
                    • Momentum is a vector quantity so it has both a SIZE and DIRECTION

11. WORK AND ENERGY

    • WORK
      • the energy used by the movement of a force is known as the WORK DONE
        • energy and work are both measure in JOULES (J) energy is the ability to do work.
          • WORK DONE (J)= FORCE (N) X DISTANCE MOVED IN DIRECTION OF FORCE (M)
            • when work is done on an object energy is transferred to that object. so amount of energy transferred (J)= work done (J)
              • energy from our food and drink is transferred so we can move and do exercise. but not all energy is transferred as work, some is lost as HEAT and SOUND energy.
    • POTENTIAL ENERGY
      • when you lift an object you do work AGAINST gravity.
        • 1 J of work will lift a weight of 1N a distance of 1m
          • the work is transferred a GPE (gravitational potential energy)
            • as an object is raised its GPE increases. Ad t falls its GPE decreases
            • CHANGE IN GPE (J)= WEIGHT (N) X VERTICAL HEIGHT DIFFERENCE (M)
              • e.g. if your weight is 700N and you climb a stairs of 3m: weight (700) x height (3)= 2100 J
    • KINETIC ENERGY
      • when you move an object to increase its velocity (get it moving) you do work. the work is transferred as KINETIC ENERGY (KE)
        • the greater the MASS=the faster the SPEED= the greater the KE
          • KE (J)= 1/2 x mass (KG) x (velocity)2 (m/s2)
            • the work done by a force is also the SAME as the change in Kinetic Energy of an object.
    • ENERGY TRANSFERS
      • as a roller coaster travels around a track, going up and down the energy changes from kinetic energy to GPE.
        • when there is NO RESISTANT FORCE the total energy remains constant. This is the PRINCIPLE OF CONSERVATION OF ENERGY
          • energy can neither be created or destroyed it can only be TRANSFERRED
            • energy is normally used up by doing work against friction and air resistance- so some energy is dissapated as heat. When an object falls, its GPE are transferred to KE. Ignoring energy transferred due to friction and air resistance: LOSS IN GPE=GAIN IN KE