forces (unfinished)

• vector quantities have


  magnitude and direction
• scalar quantities have


  magnitude and no direction
• examples of vector quantities


  force, velocity, displacement, acceleration, momentum
• examples of scalar quantities


  speed, distance, time, mass, temperature
• a force is

  A push or pull exerted on an object
• an interaction pair is


  A pair of forces that are equal and opposite and act on two interacting objects
• gravitational force is

  the force of attraction between masses
• gravity

  - attracts all masses

  - gives everything a weight
• weight is measured with

  spring balance or NEWTONMETER
• weight acts from


  centre of mass
• mass and weight are


  directly proportional
• air resistance is a contact force because


  there are collisions between the air particles and the object
• pressure is


  force per unit area
• pressure equation

  P=F/A
• pressure in liquid depends on

  depth and density
• density and pressure (liquids)

  more dense a liquid is, more particles in certain space, more collisions
• depth and pressure (liquid)

  increased depth mean more particles above a point, meaning weight is felt as pressure
• densitry and pressure equation

  P= h (p, rho) g
• upthrust

  - when object submerged (partly or fully) in liquid, pressure of fluid exerts a force on it from every direction

  - pressure increases with depth, so the bottom of the object receives more pressure than the top

  - this causes a resultant force called upthrust
• object floats if weight is equal to upthrust
• density and upthrust

  - if upthrust is equal to weight then forces balance and object floats

  - so whether an object will float depends on its density

  - an object less dense than the fluid displaced means that it weighs less than the equivalent volume of fluid displaced

  - if object is denser than fluid its displacing, it will not be able to displace a larger amount of fluid than its weight
• atmospheric pressure

  the pressure exerted by atoms and molecules in the atmosphere surrounding Earth, resulting from collisions of these particles with objects
• why does atmospheric pressure decrease with altitude/height

  - as altitude increases, atmosphere gets less dense, as fewer air molecules collide with surface

  - fewer air particles above a surface as height increases. means less weight above a point exerting pressure downwards, leading to less atmospheric pressure
• momentum is


  mass x velocity
• momentum equation


  p=mv
• in a closed system, the total momentum before an event is


  equal to the total momentum after
• momentum before


  equals momentum after (closed system!)
• conservation of momentum can be used to

  calculate velocities or masses
• forces can cause a change in momentum


  as F=ma , a=v-u/t

  so, F=change in momentum/change in time
• force causing change is equal to

  rate of change of momentum
• if someones momentum changes very quickly


  the forces acting on their body will be bigger
• longer it takes to change momentum leads to a


  smaller rate of change of momentum and the smaller the force
• examples of safety features for reducing rate of change of momentum

  cars:

  - crumple zones increase time taken for car to stop

  - seat belts that stretch slightly increasing time taken for wearer to stop

  - air bags that inflate and ensure you dont hit the dahsboard which wont slow your change in momentum

  helmets: have crushable layer of foam to lengthem time for head to stop in a crash

  - crash mats and cushioned flooring: increase time taken for you to stop when falling as material is compressible