Speed, Density & Pressure Flashcards

All 7 cards in this deck

What is the formula for speed?

speed $=$ distance $\div$ time.
So distance $=$ speed $\times$ time and time $=$ distance $\div$ speed.

What is the formula for density?

density $=$ mass $\div$ volume.
So mass $=$ density $\times$ volume and volume $=$ mass $\div$ density.

What is the formula for pressure, and what units is it measured in?

pressure $=$ force $\div$ area, measured in newtons/$\text{m}^2$ (force in newtons, area in $\text{m}^2$).

What are the steps to find the average speed for a journey made of two stages?

e.g. 40 km in 1 hour, then 20 km in 30 minutes

1. Total distance: $40 + 20 = 60$ km
2. Total time in consistent units: $1 + 0.5 = 1.5$ hours
3. Average speed $=$ total distance $\div$ total time $= 60 \div 1.5 = 40$ km/h

True or false? If one stage of a journey is driven at 80 km/h and the next at 60 km/h, the average speed for the whole journey is 70 km/h.

False. Average speed $=$ total distance $\div$ total time; the mean of the speeds only works if the two stages take the same time.

True or false? Mixing a liquid of density $70\ \text{kg/m}^3$ with a liquid of density $280\ \text{kg/m}^3$ gives a mixture of density $350\ \text{kg/m}^3$.

False. Densities cannot be added; find the total mass and total volume, then divide.

What are the steps to find the area of the face in contact from the force and the pressure?

e.g. a box exerting a force of 180 newtons at a pressure of 90 $\text{N/m}^2$

1. Start from $\text{pressure} = \dfrac{\text{force}}{\text{area}}$
2. Rearrange: $\text{area} = \dfrac{\text{force}}{\text{pressure}}$
3. Substitute: $180 \div 90 = 2\ \text{m}^2$