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Physics
Relative velocity

Look at this image showing how to calculate relative velocity for objects a) travelling in
different directions, and b) in the same direction.

● Can you see why head-on collisions are so dangerous? (That’s why you must always wear a
seat belt!)
● Can you also see why a car being overtaken by a faster car appears to be moving slowly?
textbooks nis edu kz
How to calculate relative velocity
v v = v + v v
1 OTH 1 2 2

v v = v – v v
1 OTH 2 1 2

INV Explore the motion of a steel ball in a transparent tube filled with a thick substance
like glycerine or honey. Design an investigation to allow you to calculate the speed of the
ball as it sinks down through the tube. Think!

Which variable(s) should be:
a) changed? (independent variable)

b) measured? (dependent variable)

c) kept constant? (constant variables)

OVER TO YOU!

1. A cheetah’s top speed is 120 km/h. It is only 3. The cruising speed of a standard passenger airplane
able to maintain its top speed for 12 seconds carrying 200 people is around 750 km/h. How long
before its body overheats. What is the total would it take for this airplane to go around the
distance a cheetah can cover in that time? Earth’s Equator? (The Equator is 40 000 km long.)
What assumptions are you making? Is your Give your answers in both hours and seconds.
value likely to be true? Explain your answers. 4. A swimmer moves at 1.3 m/s relative to the water,

2. A charity race over different distances is held while the speed of the river flow is 1 m/s. Determine
every year in Almaty. The first runner runs at the swimmer’s speed relative to an observer on
3.4 m/s for 15 minutes; the second runner the riverbank when she swims with the flow. What
covers 3 km in 20 minutes; and the third about when she swims against the flow?
runner covers 3.02 km distance at a speed of
5. Design a webpage to explain to students in Grade 6
8.24 km/h. Compare their speeds. Which of
the similarities and differences between speed
them covers the greatest distance?
and velocity.
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