Forces and Energy

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  • Section 20 - Forces and Energy
    • Energy transfers and systems
      • Energy is transferred between stores mechanically, electrically, by heating or by radiation
        • Kinectic, thermal, chemical, gravitational potential, elastic potential, electrostatic, nuclear, magnetic
      • Energy is transferred between stores and objects in system
        • Energy can be dissipated in wasteful ways too e.g. heat
      • A closed system is defined so that the net energy change is 0
      • If the system is a pan of water and camping stove, energy is transferred from the chemical store of the gas to the thermal store of the pan and water, increasing the temperature
      • If the system is a box being lifted, energy is transferred to its kinetic and potential energy stores
      • If the system is a toothbrush, energy is transferred from chemical store of the battery to the kinetic stores of the bristles. Energy is also dissipated by heat and sound.
    • Work Done and Power
      • Power = rate of energy transfer
      • The larger the power of an object, the more work it does per second
      • If a force moves an object, work is done
        • Energy is transferred mechanically from one store to another
    • Forces
      • Contact forces when the objects are touching each other
      • Non-contact forces have interacting fields but aren't touching
      • Two objects feel an equal but opposite force, called an interaction pair, that can be shown by arrows
      • Free-body force diagrams show all the forces acting upon an object or system with arrows of size relative to their magnitudes
      • A resultant force is the overall force on a point or object
    • Forces and Vector Diagrams
      • Scale Drawings
        • Draw all the forces acting on an object "tip to tail"
          • Then draw a straight line from the first force to the end of the last force - this is the resultant force
            • Measure the length to find the magnitude and the angle to find the direction
      • If all of the forces on an object combine to give a resultant force of 0, the object is in equilibrium
      • You can split a force into components and measure each one to measure the overall force

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