Chapter 4

• Define centre of gravity


  An imaginary point through which the entire weight of an object appears to act. Can be determined through the use of a plum line
• Define centre of mass

  A point through which any externally applied forces produces lateral motion only and no rotation
• weight is the gravitational force acting on an object through its centre of mass
• friction is the force that arises when two surfaces rub against each other
• drag is the resistive force on an object travelling through a fluid, the same as friction
• tension is the force within a stretched cable or rope
• upthrust is a upward forces acting on an object when it is in a fluid
• normal contact force is the force arising when one object rests against another object
• Define terminal velocity


  The constant speed reached by an object when the drag force is equal and opposite to the weight of the object
• Forces on a falling object

  The instant an object starts to fall there is no drag force on the object. The total force is equal to the weight. The acceleration of the object is g, the acceleration of free fall.

  As the object falls, its speed increases and this in turn increases the magnitude of the opposing drag force. The resultant force on the object decreases and instantaneous acceleration is less than g.

  When the object reaches terminal velocity, when the drag force on the object is equal and opposite to its weight. At terminal velocity, the object has zero acceleration
• Define moment

  The product of force and perpendicular distance from a pivot or stated point
• Principal of moments

  For a body in rotational equilibrium, the sum of the anticlockwise moments about any point is equal to the sum of the clockwise moments about that same point
• A couple of forces is a pair of forces that act in the same direction along two different parallel lines
• The moment of a couple is known as a torque.
• Define density

  the mass per unit volume
• Define pressure

  The force exerted per unit cross-sectional area
• Define upthrust


  $Ax\rho g$
• Archimedes' principle states the upthrust exerted on a body immersed in a fluid is equal to the weight of the fluid displaced