Speed, Pressure and Density

Understanding Speed, Pressure and Density

• Speed relates distance travelled to the time taken. It’s measured in metres per second (m/s), kilometres per hour (km/h), or miles per hour (mph).
• Calculations involving speed use the formula: speed = distance ÷ time. Ensure you’re working with compatible units.
• Pressure is the force applied per unit area. It’s often measured in pascals (Pa), or sometimes in atmospheres (atm), millimetres of mercury (mmHg), or pounds per square inch (psi).
• Pressure is calculated using the formula: pressure = force ÷ area. The units of force and area must be compatible.
• Density expresses how much mass is in a given volume. Density is commonly measured in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³).
• Use the formula: density = mass ÷ volume when working with densities. Ensure all units are compatible.

Applying Speed, Pressure and Density Concepts

• To apply the concepts practically, you must identify necessary quantities in a given problem and substitute these values into the formula.
• Units must always be compatible. Convert units where necessary before substituting them into the formulae.

Plotting and Interpreting Graphs for Speed, Pressure and Density

• Plotting distance-time graphs provides a way to visualise an object’s speed. A steeper slope on the graph indicates a greater speed.
• Plotting force-area graphs can visualise pressure. A steeper slope represents greater pressure.
• Understanding the relationship between mass and volume can be reached by plotting a mass-volume graph. The steeper the line, the greater the density.

Dealing with Inverse Proportions in Speed, Pressure and Density

• Pressure is inversely proportional to area when the force is constant. If the area doubles, the pressure halves, assuming the force remains constant.
• Similarly, speed is inversely proportional to time when covering the same distance. If the time taken doubles, the speed is halved, if the distance travelled remains the same.

Solving Real-life Problems Involving Speed, Pressure and Density

• These principles of speed, pressure and density are often used to solve real-life problems, like finding the speed of a car, the pressure under water, or the density of an object.
• Recognise when to use each formula and ensure your solutions are reasonable in the context of the problem. For instance, a negative value for speed, pressure, or density in most physical settings is not meaningful.