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Units of motion for SQA National 5 Physics

Units of motion

This page covers the following topics:

1. Units of distance
2. Units of speed
3. Units of acceleration
4. Proving units

The main SI unit for measuring distance is metre and the symbol used for it is m. Other common units (and their symbols) used for measuring distance are millimetres (mm), centimetres (cm) and kilometres (km).

Units of distance

The main SI unit for measuring speed is metres per second, given as m/s or msโปยน. An alternative unit for speed would be kilometres per hour, given as km/h or kmhโปยน. Speed is a compound quantity, as it is made up of both distance and time, therefore to convert between units of speed both distance and time must be converted separately.

Units of speed

The SI unit for measuring acceleration is metres per second squared, given by m/sยฒ or msโปยฒ. An alternative unit that can be used is km/hยฒ or kmhโปยฒ. Just like speed, acceleration is a compound quantity made up of distance and time, therefore when converting between units of acceleration, the units for distance and time must be converted separately.

Units of acceleration

The equations of speed and acceleration can be used to prove the units used for distance, speed and acceleration. Speed and acceleration are both compound quantities, therefore by using the units of the separate quantities that make up their equations, their units can be found.

Proving units

1

Find the main SI units of speed using the speed equation.

Using speed = distance/time and measuring distance in m and time in s, speed = m/s.

m/s

Find the main SI units of speed using the speed equation.

2

Convert 5 m/s to km/h. Give your answer in an exact form.

5 m/s = (5/1000) ร— 3600 s = 18 km/h

18 km/h

Convert 5 m/s to km/h. Give your answer in an exact form.

3

Convert 8.5 m/sยฒ to km/hยฒ.

8.5 m/sยฒ = 8.5/1000 ร— 3600ยฒ km/hยฒ = 110160 km/hยฒ

110160 km/hยฒ

Convert 8.5 m/sยฒ to km/hยฒ.

4

A runner runs 14.2 metres in 2 seconds. Find the speed of the runner in km/h.

speed = distance/time
speed = 14.2/2 m/s = 7.1 m/s
speed = (7.1/1000)/(1/3600) km/h = 25.56 km/h

25.56 km/h

A runner runs 14.2 metres in 2 seconds. Find the speed of the runner in km/h.

5

Give 16.4 km/hยฒ in m/sยฒ to 3 significant figures.

16.4 km/hยฒ = (16.4 ร— 1000)/3600ยฒ m/sยฒ = 0.00127 m/sยฒ (to 3 significant figures)

0.00127 m/sยฒ

Give 16.4 km/hยฒ in m/sยฒ to 3 significant figures.

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