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Tuesday, February 9, 2016


Speed measures how far we travel in a unit of time.  This is why we measure it in miles per hour.  Miles measure distance per means in every and hours measure time.  Of course there are other units of speed as well such as kilometres per hour and meters per second which we use a great deal in common entrance physics. Fast and slow moving things might need different units. A rocket will move a certain number of kilometres per second (around 11 km/s to escape earth orbit) a glacier might move a few
centimetres/year. It would be silly to measure these movements in meters per second the numbers would be really big or really small and any calculations you do complicated.
So to measure speed we need to know the distance moved and the time taken. Then we divide the distance by the time. In other words:
Speed= distance / time
If you forget this just think of a speed you go in your car. 50 miles per hour. Miles (distance) per (divided by) hours (time).

I am on a train as I write this we are going 60 miles in 2 hours so the speed is
S=60/2=30 mph

Remember this will be our average speed. We stop at each station, speed up and slow down all the way so we won't go at 30mph exactly much of the time.

If my train was travelling for 4 hours at this average speed how far would it go?
Well the distance=speed x time
Distance =30 x 4 =120 miles.
This is because the speed tells us the distance we go each hour if I go 4 lots of 30 that is 120 miles.
So how long would it take my train to do 105 miles?
Here we do time=distance/speed
Time=105/30=3.5 hours.
Take care. Exam question setters love to put in traps. Look at this question.
A horse travels at 8m/s for two minutes how far does it go?
Here I have muddled up minutes and seconds. The first thing we must work out is how many seconds the horse is travelling for. 2x60=120 (as there are 60 seconds in one minute)
Now we can do
distance=speed x time
Distance=8 x120

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