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 (vav) for a journey is calculated from the distance travelled divided by the time taken to travel that distance:

Average Speed = distance travelled or vav = 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 Unit Time Unit Speed Unit

centimetre cm second s centimetre per second cm s-1

metre m second s metre per second m s -1

kilometre km hour h kilometre 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

Vav = 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.