## 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.

The turning effect 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**.

**moment of a force = force × distance**

moment of a force = 1400 x 0.3

The moment of the rock is **420 Newtons**

Law of MomentsIf an object is balanced, the

Learn this definitiontotal clockwise moment about a pivot equals the total anticlockwise momentabout that pivot.

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

**total clockwise moment about a pivot = the total anticlockwise moment**

300 x 2.0 = mass of second person x 1.5

mass of second person = 600 / 1.5

The mass of the second person is **400 Newtons**

## 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 m^{2}.

Calculate the pressure exerted by the water on the bottom of the container.

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

pressure = 27 / 0.009

The pressure is **3000 Pascals**

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

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

pressure change = (20 – 8) x 1000 x 9.8

pressure change is** 117600 Pascal**

**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)

rate of flow of water less

because pressure is less

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.

air molecules colliding with a surface create pressure

at increasing altitude fewer molecules

so always less weight of air than below