Forces in Balance

• Displacement


  The distance in a given direction.
• A vector quantity is a physical quantity which has magnitude and direction.
• A scalar quantity has magnitude but no direction.
• A vector quantity can be represented by an arrow in the direction of the vector and of length in proportion to the magnitude of the vector.
• Forces can change the shape of an object, or change its motion or its state of rest.
• The unit of force is the newton (N).
• When two objects interact, they always exert equal and opposite forces on each other.
• The resultant force is a single force that has the same effect as all the forces acting on an object.
• If the resultant force on an object is:

  • Zero, the object stays at rest or at the same speed and direction
  • Greater than zero, the speed or direction of the object will change.
• If two forces act on an object along the same line, the resultant force is:

  • Their sum, if the forces act in the same direction
  • Their difference, if the forces act in opposite directions.
• A free-body force diagram of an object shows the forces acting on it.
• The centre of mass of an object is the point where its mass can be thought of as being concentrated.
• The centre of mass of a uniform ruler is at its midpoint.
• When an object is freely suspended, it comes to rest with its centre of mass directly underneath the point of suspension.
• The centre of mass of a symmetrical object is along the axis of symmetry.
• The parallelogram of forces is a scale diagram of two force vectors.
• The parallelogram of forces is used to find the resultant of two forces that do not act along the same line.
• The resultant is the diagonal of the parallelogram that starts at the origin of the two forces.
• Resolving a force means finding perpendicular components that have a resultant force that is equal to the force.
• To resolve a force in two perpendicular directions, draw a rectangle with adjacent sides along the two directions so that the diagonal represents the force vector.
• For an object in equilibrium, the resultant force is zero
• An object at rest is in equilibrium because the resultant force on it is zero.