forces topic revision

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  • forces
    • vectors and scalars
      • vectors-like displacementanything we can measure in science that has a direction is a vector.
        • examples: displacement velocity acceleration, force
        • vector quantities have magnitude and a specific direction
        • Untitled
        • vectors: 800m east, 50N of force, 1830m north.
      • scalars-if something can be measured without a direction it is called a scalar.
        • examples: distance, speed, mass, temperature
        • scalar quantities have size or magnitude but a direction is not specified
        • scalars: 45 minutes, 50g, 175ml, 30/40 minutes.
    • work done
      • one way of saying “zero results at force” is “balanced forces”
        • balanced forces - no change in speed or direction (could remain stationary)
      • one way of saying “resultant force bigger than  zero” is “unbalanced forces”
        • unbalanced forces-speed changes (accelerate or decelerate)
      • work done = distance x force (wet foxes stink)
    • contact and non-contact forces and resultant force
      • contact forces: friction, drag, tension, thrust
        • if two objects have to touch each other to feel a force the forces are contact forces.
      • non-contact forces: magnetism, static electricity, gravity
        • if two objects feel a force without touching the forces are non-contact
      • newton’s third law: “when two objects interact with each other, they exert equal an opposite forces on each other” (push and pull)
    • gravity and centre of mass
      • weight is measured in newton’s and measured using a newton metre
      • centre of mass - point where the weight of an object is considered to act
        • for a symmetrical object the centre of mass is along the centre of symmetry
        • for a irregularly shaped object you must suspend the shape attach a plumb line ( a piece of string with a mass on the end) and allow it to fall to rest and draw a line down the shape using aboard pen. The centre of mass will be on this line. Suspend the object from a different point and repeat the process and where the lines cross is the centre of mass.
      • gravity
        • Weight is the force acting on an object due to gravity. The force of gravity is close to the earth due to the gravitational field around the earth.
        • The weight on an object depends on the gravitational field strength at the point where the object is.
        • weight = mass x gravitational field strength  (worms munch garbage)
          • weight and mass are usually directly proportional
    • forces and elasticity
      • required practical: see date Monday 1st June 2020 for diagram
        • work out the extension produced by adding each weight.
          • to do that we subtract the length of the unstretvhed spring from each new reading.
      • inelastic deformation, limit of proportionality.
      • straight line through 0 is directly proportional, linear graph created
      • Elastic potential energy: the energy stored in anything that is stretched or squashed. The amount of energy depends on how much the thing is stretched or squashed. We call this the extension which is measured in metres. to find the extension you do : extension = new length- original length. The elastic potential energy also depends on the spring constant which tells us how easy or difficult it is to strength the thing.
    • required practical-stretching a spring
      • attach two clamps to a clamp stand and attach a spring to the top clamp and a ruler to the bottom clamp
        • move the ruler so that the 0cm mark is level with the top of the spring and record the length of the spring
          • hang a 100g mass on the spring and measure the new length of the spring
            • calculate the extension (stretched length-original length)
              • repeat until you have added a total of 500g
      • when you first stretch the spring, the extension and force are proportional
      • f=k x e (french kangaroos exist)- Robert Hooke
    • parallelogram and resolution of forces
      • parallelogram
        • a parallelogram is a simple four-sided shape with two pairs of parallel sides
        • the opposite of facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are the same size too
      • zip wire model: use a length of thin string and a weight hanger or other suitable object to make and test the model
        • release the weight hanger on the string at the top and observe be where it comes to rest.
          • investigate how the height difference between the ends of the string affects the horizontal difference between the rest position of the hanger to one of the stands.
      • resultant force : F3 = F1 + F2

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