Car at national speed limit Passenger plane Speed of sound
Earth moving around the Sun Speed of light in a vacuum
EXAMPLE
Question If a train is moving at a constant speed of 30 m/s, calculate: a. how long it takes to travel 2.1 km b. how far it would travel at this speed for 10 minutes
Solution
a. time = distance (m) speed (m/s)
time = 2100 m 30 m/s
time = 70 s (1 minute 10 seconds) :Fig. 27.4
b. distance = speed × time distance = 30 m/s × (10 × 60 s) distance = 30 m/s × 600 s distance = 18000 m (18 km)
Speed (m/s) 1.5 8
27.7 250 330
30000 300000000
Eureka
The speed of light is the fastest thing in the universe. The time it takes for light to travel from the Moon to Earth is 1.3 seconds – a distance of nearly 390000 kilometres!
CONCENTRATE! The following triangle is a useful tool for remembering the formulae for calculating distance, speed and time. Simply cover the quantity you want to measure with your finger and use the formula that you can still see.
? TEST TUBE
1. Usain Bolt set a world record by running 100 m in 9.58 s. a. Calculate his average speed in m/s. b. Do you think he can go faster than this? Explain your answer.
2. One of the fastest dogs in the world is the Dobermann. It can maintain a speed of over 50 km/h.
a. Calculate how long it would take the Dobermann to run 100 m.
b. Look up other animals and see how fast they can run compared to humans.
:Fig. 27.5
distance speed × time
FIGURE IT OUT!
1. A vehicle on a motorway travels 1900 m in 60 s. Calculate: a. the average speed of the vehicle in m/s b. how far it would travel at this speed for 2 minutes c. how long it would take to travel a distance of 2 km at this speed