Physics - Forces Revision

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  • Forces
    • Scalars and Vectors
      • The difference between Scalars and Vectors is that Vectors must have a specific direction, whereas Scalars don't.
      • Three examples of Scalars are distance, speed, mass.
      • Three examples of Vectors is force, velocity, displacement.
      • Vectors have magnitude and direction.
      • Scalars have only magnitude and no direction.
      • Vectors are usually represented by an arrow.
        • The length of the arrow shows the magnitude and the direction of the arrow shows the direction of the quantity
    • Contact and Non Contact Forces + Resultant forces
      • Contact Forces - two objects have to touch each other to feel a force
        • An interaction pair is a pair of forces that are equal and opposite and act on two interacting objects.
      • Non Contact Forces - two objects feel a force without touching.
      • If you push or pull something, you make a pair of forces.
        • They are the same size but with different directions.
      • A resultant force is the overall force on a point or object.
        • If you have a number of forces acting on a single point, you can replace them with just one force.
          • This single force is called the resultant force.
        • If the forces all act along the same line, the overall effect is found by adding those going in the same direction, and subtract those going in the other direction.
    • Work done
      • When a force moves an object through a distance, energy is transferred and work is done on the object.
        • Whether energy is transferred usefully or is wasted, you can still say "work is done"
      • You can find out how much work has been done by using an equation.
        • W=fs        Work done = Force x Distance
    • Gravity and Centre of Mass
      • Gravitational force is the force of attraction between masses.
      • Weight and mass are not the same.
        • Mass is just the amount of "stuff" in an object.
          • Mass is not a force. It is measured in kilometres.
        • Weight is the force acting on an object due to gravity.
          • Close to earth, this force is caused by the gravitational field around earth.
            • Gravitational field strength varies with location.
          • The weight of an object, depends on the strength of the gravitational field.
            • For example, on earth and the moon a person would have the same mass but not the same weight.
            • Weight is a force measured in Newtons.
    • Required practical: stretching a spring.
      • 4 simple steps :          1. Measure the natural length of the spring with a millimetre ruler.            2. Add a mass to the spring and allow it  to come to rest. Measure the new length of the spring.   3.Repeat this process until you have enough measurements.                  4. Plot a graph of your results.
    • Forces and Elasticity.
      • Hooke's Law:  Force = Spring Constant x Extension
      • Work done in stretching a spring : elastic potential = 0.5 x spring constant x (extension)squared.

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