Page 82 - York Global Science 6 - Physical Science

But motion is relative. It depends on the frame of reference that you are

using. If you are a passenger on the train, the things inside the train are not
moving! For example, the cup of coffee is stationary and the other passengers
are stationary, but the objects outside the train appear to be moving.

We often describe motion using the Earth as a frame of reference. We can also
compare the relative motion of two moving objects. Look at these cars.

Both of these cars are moving quickly in the same direction. Soon, the red car
will move in front of the blue car because it is moving at a faster speed.
A person standing on the side of the

road, using the Earth as a frame of
reference, notes the speed of each
car as 50km/h and 70 km/h.

However, the driver of the blue car is
using his car as a frame of reference. 50 km/h
When the red car goes past him, it

looks like the red car is going much 70 km/h
slower than 70 km/h. This is because
the frame of reference he is using is also moving.

For the man driving the blue car, it looks like the red car is only moving at 20
km/h:

70 km/h - 50 km/h = 20 km/h
The relative speed of the red car to the blue car is 20 km/h.

If the cars are moving in opposite directions, the relative speed increases. The
driver of each car sees a much higher speed than the real speeds that the
cars are moving. This is because the cars are moving away from each other.

50 km/h

70 km/h

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