forces

• Engineers analyse forces when designing a great variety of machines and instruments, from road bridges and fairground rides to atomic force microscopes. Anything mechanical can be analysed in this way. Recent developments in artificial limbs use the analysis of forces to make movement possible.
• Scalar quantities


  Have magnitude only
• Vector quantities


  Have magnitude and an associated direction
• Vector quantity representation


  An arrow, the length represents the magnitude, the direction represents the direction of the vector quantity
• Contact forces


  The objects are physically touching
• Contact forces


  • Friction, air resistance, tension, normal contact force
• Non-contact forces


  The objects are physically separated
• Non-contact forces


  • Gravitational force, electrostatic force, magnetic force
• Force is a vector quantity
• Interaction between pairs of objects producing a force


  Forces represented as vectors
• Weight


  The force acting on an object due to gravity
• Gravitational field strength


  The force of gravity close to the Earth
• The weight of an object may be considered to act at a single point referred to as the object's 'centre of mass'
• The weight of an object and the mass of an object are directly proportional
• Newtonmeter


  Used to measure weight
• Resultant force


  A single force that has the same effect as all the original forces acting together
• A single force can be resolved into two components acting at right angles to each other. The two component forces together have the same effect as the single force.
• One joule of work is done when a force of one newton causes a displacement of one metre.
• Work done against the frictional forces acting on an object causes a rise in the temperature of the object.
• Elastic deformation and inelastic deformation

  • Caused by stretching forces
• If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
• Fluid


  Can be either a liquid or a gas
• A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.
• Atmospheric pressure


  Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.
• Distance


  How far an object moves, does not involve direction, scalar quantity
• Displacement


  Includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line, vector quantity
• Speed


  Does not involve direction, scalar quantity
• The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing.
• Typical values may be taken as: walking ̴ 1.5 m/s, running ̴ 3 m/s, cycling ̴ 6 m/s.
• A typical value for the speed of sound in air is 330 m/s.
• Walking speed


  ̴ 3 m/s
• Cycling speed


  ̴ 6 m/s
• Students should be able to recall typical values of speed for a person walking, running and cycling as well as the typical values of speed for different types of transportation systems
• It is not only moving objects that have varying speed. The speed of sound and the speed of the wind also vary
• Speed of sound in air

  330 m/s
• Students should be able to make measurements of distance and time and then calculate speeds of objects
• distance, s


  in metres, m
• speed, v


  in metres per second, m/s
• time, t


  in seconds, s
• Students should be able to calculate average speed for non-uniform motion