## 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

or

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?

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.