Chapter 5 - Moments

1.

Explain briefly how the use of a screwdriver enables the lid of a paint can to be opened with a relative small force.

  • Moment is the product of force and the perpendicular distance between the force and the pivot.
  • When a force is applied at the far end of the screw driver, the perpendicular distance between the pivot and line of action of applied force is large. This will then produce a large moment to open the paint can.

 2.

Describe an experiment to locate the centre of gravity of an irregular shaped lamina.

  • Make 3 small holes, as far apart as possible, near the edge of the card.
  • Suspend the card through one of the holes using a pin.
  • Hang a plumbline from the pin in front of the card.
  • When the plumbline is steady, draw the vertical line on the cardboard as indicated by the plumbline.
  • Repeat the above for two other holes.
  • The point of intersection of the three lines is the position of the centre of gravity of the cardboard (lamina).

3.

For any free hanging object, the centre of gravity must be vertically below the pivot. Explain.

  • When the CG is vertically below the pivot, the perpendicular distance between the line of action of the weight to the pivot is zero.
  • Since the perpendicular distance between the force and the pivot is zero, the resultant moment will also be zero and the object does not turn.

 4.

How can an object be made more stable?

  • Lower its centre of gravity by making the base heavy.
  • Increase its base area.