Motion

Distance and Time Motion is described using words like distance, speed, time and acceleration. Time (t) is measured in seconds (s). A time of 14 seconds is written t = 14 s. Other units of time are minute (min) and hour (h).

Distance (d) is measured in metres (m). A distance of 28.5 metres is written d = 28.5 m. Other units of distance are cm and km.

Speed The average speed (v av ) for a journey is calculated from the distance travelled divided by the time taken to travel that distance: Average Speed = distance travelled or v av = d time taken t

Instantaneous speed is the actual speed at any moment. – Eg: the speedometer in a car measures the instantaneous speed.

If the instantaneous speed does not change, then the speed is described as being constant, uniform or steady. Units of speed; Distance UnitTime UnitSpeed Unit centimetre cmsecond scentimetre per second cm s -1 metre msecond smetre per second m s -1 kilometre kmhour hkilometre per hour km h -1

Formula When using a formula to make a calculation, always write down: – The formula being used. – A working step showing the numbers in the calculation. – The answer with the correct units.

How fast are you? Go out on to the field. Measure 10 metres and time how long it takes you to run/walk. Use a stopwatch and record your time. In class use the formula to work out your speed. Remember; Speed = distance/time

V av = d/t The above formula can be used to calculate; – The distance travelled, d,when v and t are known. – The time taken, t, when d and v are known.

Examples 1.A car is travelling at a constant speed of 30 m s -1. How far does it travel in 40 seconds? 2.A snail is moving along a cabbage leaf at 0.1 cm s -1. What time does it take to travel 3 cm?

Solutions 1.d = vt = 30 x 40 = 1200 m or (1.2 km) 2.t = d/v = 3/0.1 = 30 s

Exercises. What is the length of the objects shown? – A – B – C

More Exercises Complete ‘Speedy Problems’ on page 9 and 10 of your workbook.