Moments, levers and pressure in fluids

Moments

• A force or a system of forces may cause an object to rotate. The turning effect of a force is called its moment.
• M = F d: moment (N m) = force (N) × perpendicular distance from the pivot to the line of action of the force (m).
• If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot (the principle of moments).

Levers and gears

• Levers and gears transmit the rotational effects of forces.
• A lever increases the moment by increasing the distance from the pivot, so a small force can move a large load (for example a crowbar or a spanner).
• A small gear driving a larger gear makes the larger gear turn more slowly but with a bigger moment.

Pressure in fluids

• A fluid is a liquid or a gas. The pressure in a fluid causes a force at right angles (normal) to any surface.
• p = F ÷ A: pressure (Pa) = force normal to a surface (N) ÷ area of that surface (m²).
• (Higher) The pressure due to a column of liquid: p = h ρ g, where h is the depth and ρ the density. Pressure increases with depth and with the density of the liquid.

Upthrust and floating (Higher)

• An object partly or totally submerged in a fluid experiences a greater pressure on its bottom surface than on its top surface. This creates a resultant upward force called upthrust.
• An object floats if its weight is equal to the upthrust. It sinks if its weight is greater than the upthrust.
• Objects less dense than the fluid float, because they can displace a weight of fluid equal to their own weight.

Atmospheric pressure

• The atmosphere is a thin layer of air around the Earth; air molecules colliding with surfaces create atmospheric pressure.
• Atmospheric pressure decreases with height, because there is less air above (and the air is less dense).

Key terms

Moment

The turning effect of a force: force × perpendicular distance from the pivot.

Pivot

The point about which an object turns.

Principle of moments

For a balanced object, total clockwise moment = total anticlockwise moment.

Lever

A simple machine that uses a pivot to increase the turning effect of a force.

Pressure

Force per unit area: p = F ÷ A.

Fluid

A liquid or a gas.

Upthrust

The upward force on an object in a fluid, caused by the pressure difference between its top and bottom.

Atmospheric pressure

The pressure caused by air molecules colliding with surfaces.