Physics Ch: 4

Define moment of a force.

Turning effect of a force about a pivot. M = F * perpendicular distance.

Example of moment in daily life?

Force applied on a door away from hinges makes it easier to open (greater perpendicular distance).

When is turning effect greatest?

When force acts perpendicular to the lever arm.

Conditions for equilibrium of an object?

Net force = 0 and net moment = 0.

State the principle of moments.

In equilibrium, total clockwise moment = total anticlockwise moment about a point.

Define centre of mass.

Point where all mass of object is assumed concentrated.

Define centre of gravity.

Point where all weight of object is assumed concentrated.

Where is centre of mass for regular symmetric objects?

At the geometrical centre.

Why is a bottom-heavy object more stable?

Lower centre of mass, needs greater tilt angle to topple.

Why is a top-heavy object less stable?

High centre of mass, small tilt angle can topple it.

How to find centre of mass of an irregular lamina?

Hang lamina by holes, draw vertical lines with plumb line. Intersection = centre of mass.

Why is a uniform metre rule balanced at 50 cm mark?

Mass is evenly distributed; centre of mass at mid-point.

What is stable equilibrium?

When object returns to original position after small tilt (line of weight within base).

What is unstable equilibrium?

When line of weight passes beyond base, object topples.

What is neutral equilibrium?

When object stays in new position after being moved (e.g. ball on flat surface).