Moments: unit Nm (Triple Physics Only)

A force or a system of forces may cause an object to rotate.

The turning effect of a force is called the moment of the force. The size of the moment is defined by the equation:

moment of a force = force × distance

Question: State what is meant by the moment of a force.

Question: Figure 1 shows how a lever can be used to lift a heavy rock.

Calculate the moment of the weight of the rock about point P.

Law of Moments

If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.

Learn this definition

A simple lever and a simple gear system can both be used to transmit the rotational effects of forces.

Question: Figure 2 shows a person sitting at the right end of the see-saw, 2.0m from the pivot. Her weight is 300 N.
Figure 2
The see-saw is balanced.
Calculate the weight of the person who is 1.5m from the pivot

Pressure & Fluids (Triple Physics Only)

A fluid can be either a liquid or a gas.

The pressure in fluids causes a force normal (at right angles) to any surface. Pressure is measured in Pascals, Pa.

The pressure at the surface of a fluid can be calculated using the equation:

pressure = force normal to the surface / area of the surface

Question: The water exerts a force of 27 N on the bottom of the container.
The cross-sectional area of the bottom of the container is 0.009 m2.
Calculate the pressure exerted by the water on the bottom of the container.

The pressure due to a column of liquid can be calculated using the equation:

pressure = height of the column × density of the liquid × gravitational field strength


pressure change = change in depth × density of the liquid × gravitational field strength

Question: What is the change in pressure as a diver moves from a depth of 20m below the surface to a depth of 8m below the surface? (g=9.8n/kg). Density of water = 1000kg/m³

Question: The diagram shows a water butt used to collect rainwater.
A tap allows water to be collected from the water butt in a watering can.
If the tap was placed higher up on the water butt, what difference would it make to the rate of flow of water from the tap?
Explain your answer. (2 marks)

In a liquid, pressure at a point increases:

  • with the height of the column of liquid above that point; because there is more weight acting caused by the liquid above the point
  • with the density of the liquid; if the density increases the weight of the liquid above the point increases

Calculate the differences in pressure at different depths in a liquid.

A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.

An object will float if the weight of the object is less than the upthrust acting on it.

The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.

Air molecules colliding with a surface create atmospheric pressure.

The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.

Question: Explain why atmospheric pressure decreases with increasing altitude.