6.1Chapter 6: Forces, Motion and Pressure

Speed — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

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Physics 6.1 Speed — Calculating Speed, Distance and Time (v = d / t) — Alhussan International Academy — Physics (Stages 7–9) notes: Calculate speed and distinguish average speed from instantaneous speed.Speed = a measure of how fast something is moving, the distance moved per uni…

Speed equation
Rearranged forms
Why speed varies
m/s to km/h
km/h to m/s
Comparing speeds

Section 1 of 8What speed is

Speed

Speed a measure of how fast something is moving, the distance moved per unit of time.

Units

Speed is measured in metres per second (m/s) or kilometres per hour (km/h).
In the UK and USA speeds are usually given in miles per hour (mph).
A person walks at about 5 km/h or 1 m/s; a fast car drives at over 300 km/h or about 80 m/s.

Section 2 of 8Calculating speed

Speed equation

Speed = distance ÷ time (v = d ÷ t).
To find the speed of a moving object, measure the time it takes to travel a known distance.

Rearranged forms

Distance = speed × time (d = v × t) and time = distance ÷ speed (t = d ÷ v).

Units

Writing the units of distance and time into the equation gives the correct unit for speed, such as 100 m ÷ 20 s = 5 m/s.

Method

Write the equation, substitute the values with their units, calculate, and state the answer with its unit.

Section 3 of 8Average speed

Average speed

Average speed total distance ÷ total time.

It is found over a whole journey and includes stops and changes of speed.
It makes it easier to compare how fast different people, boats or cars travel.

Why speed varies

A sailing boat goes faster in strong wind and slower when there is very little wind.

Section 4 of 8Instantaneous speed

Instantaneous speed = the speed at one moment in time.
- The reading on a car's speedometer is an instantaneous speed.
- It is worked out from the size of the wheels and how fast they are turning, not from the whole journey.

Section 5 of 8Typical speeds and unit conversion

Object	Typical speed
Walking	1.5 m/s
Cycling	5 m/s
Car on a motorway	30 m/s (about 108 km/h)
Hurricane wind	over 33 m/s
Plane cruising	250 m/s
Sound in air	330 m/s

m/s to km/h

Multiply by 3.6 .
- (1 m/s = 3.6 km/h)

km/h to m/s

Divide by 3.6.

Comparing speeds

Convert both to the same unit first, because a smaller number in a larger unit can be the faster speed.

Section 6 of 8Measuring speed in an experiment

Measure a known distance with a ruler or tape measure.

Time the object over that distance with a stopwatch or light gates.

Use a long distance so that reaction-time errors are small compared with the total time.

Repeat the measurement and calculate a mean to reduce random errors.

Divide the distance by the mean time.

Section 7 of 8Key terms

Term	Meaning
Speed	The distance moved per unit of time.
Average speed	Total distance divided by total time.
Instantaneous speed	The speed at a particular moment in time.
Metre per second (m/s)	The SI unit of speed.
Speedometer	A device showing the instantaneous speed of a vehicle.