Force, Density and Pressure

• Force


  rate of change of momentum
• Forces on masses in gravitational fields


  A region of space in which a mass experiences an attractive force due to the presence of another mass.
• Forces on charge in electric fields


  A region of space where a charge experiences an attractive or repulsive force due to the presence of another charge.
• Upthrust ( Buoyancy)


  An upward force exerted by a fluid on a submerged or floating object
• Origin of upthrust


  The pressure on bottom surface is larger than that on top surface, so the force on bottom surface is. large than that on the top surface. The resultant force is thus upwards.
• Frictional force


  Forces that arises when two surfaces rub.①Always opposes relative or attempted motion② Always acts along a surface③ Value varies up to a maximum value..
• Viscous forces


  ① A force that opposes the motion of an object in a fluid that only exists when there is motion② Its magnitude increases with the speed of the object.
• Centre of gravity


  Point through which the entire weight of the object may be considered to act.
• Couple


  A pair of forces which produce rotation only
• To form a couple, the two forces must be


  ① Equal in magnitude② parallel but in opposite directions③ separated by a distance
• moment of a force


  Product of the force and the perpendicular distance of its line of action to the pivot.
• Torque of a couple


  The product of one of the forces of the couple and the perpendicular distance between the lines ot action of the forces.
• Conditions for Equilibrium

  ① Resultant force acting on it in any direction equals zero② Resultant torque about any point is zero.
• Principle of Moments


  For a body to be in equilibrium, the sum of all the anticlockwise moments about any point must be equal to the sum of all the clockwise moments about that same point.
• Density


  the mass per unit volume of a substance
• Pressure


  The normal force per unit area