Forces and moments

• Force is the rate of change of momentum
• Forces on masses on 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 a force due to the presence of another charge
• Origin of upthrust

  • Pressure on the bottom surface > pressure on the top surface
  • Therefore, the force on the bottom surface > force on the top surface
  • ~resultant force upwards
• Frictional force - a force that opposes the motion when the surface of a body comes into contact with another body
• Viscous force - a force that opposes the motion of an object in a fluid
• Drag force - a viscous force acting against the relative movement between two objects
• Terminal velocity - the maximum speed at which an object falls through a fluid under gravity
• The terminal velocity is reached when the drag force equals the weight of the object
• Moment - product of the force and the perpendicular distance of its line of action to the pivot
• Moment = Force x Distance from pivot
• Equilibrium - the condition o a system when neither its state of motion nor its internal energy state tends to change with time
• For an object to be in equilibrium, both the resultant force and the resultant moment acting on the object must be zero
• Principle of moments - for a body in equilibrium, the sum of the anticlockwise moments about any point must be equal to the sum of the clockwise moments about that same point
• A couple is a pair of forces which have different lines of action but act at the same point
• Couple - a pair of coplanar forces, where the two forces are equal and opposite parallel forces that act on an object through different lines of action. It has the effect of causing a rotation with the translation
• Torque - the turning effect of moment of a couple
• To form a couple,

  • equal in magnitude
  • parallel but in opposite directions
  • separated by a distance