Speed or Velocity? Get it right.
This is a sloth. According to Wikipedia
its maximum velocity is only 0.24 km/h.
Covert this to m/s ………………………………
……………………………………………………………
……………………………………………………………
……………………………………………………………
At its maximum velocity how long
would it take for it to have a
displacement of 1 km?
…………………..
…………………………………………………………….
…………………………………………………………….
…………………………………………………………….
Physicists like to be very precise with their language. For example in everyday life the words speed
and velocity can mean the same thing. To a physicist they are completely different. Speed is a scalar
quantity and velocity is a vector quantity.
A scalar quantity is one that has size (physicists prefer the word “magnitude”) only. Examples of
scalar quantities are mass, energy and temperature.
Vector quantities have size (magnitude) but they also have a direction. An example of this is force
(measured in newtons). The force has a size of perhaps 300 N but it also has a direction for example
downwards or to the left or to the right.
Distance is a scalar quantity – it tells us how far something has moved (for example the odometer in
a car tells us how much distance the car has travelled since it was made).
Displacement is the vector equivalent of distance. It tells us how far something has moved in a
particular direction. If the object moves in the opposite direction velocity can even be negative!
Speed is a scalar quantity. It tells us about how much distance has been travelled per second or per
hour etc.
Velocity is a vector quantity. It tells us about how much displacement there has been per second or
per hour etc.
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Notice the equation we use for velocity and how it is different from the equation we use for speed.
Fun question: Are the following vectors or scalars?
Mass, Weight, Temperature, Acceleration, Hong Kong Dollars
Speed =
distance travelled
time taken
Velocity =
displacement
time taken
Key Facts
s = distance or displacement
t = time taken
v = speed or velocity
S
Note; displacement is the distance travelled in a
given direction
÷
V
x
t
What is a;
1. Scalar………………………………………………………………………………………………………………………………………………………
2. Vector…………………………………………………………………………………………………………………………………………………….
Sort the quantities into vectors or scalars and complete the table below.
Displacement
Speed
Distance
Velocity
Vector
Scalar
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Mr. Corelli’s Journey
Displacement is always the shortest distance between two points.
Scale: 1 cm = 2 km
Mr. Corelli lives near Ngau Chi Wan station and regularly travels by MTR.
1) What distance does he travel if he goes from Ngau Chi Wan station to Lai Chi Kok station? (you may
want to use a piece of string to help you calculate this)
2) What is his displacement when travels from Ngau Chi Wan to Lai Chi Kok?
3) He now travels from Ngau Chi Wan to Austin Road station and then to Lai Chi Kok. Calculate both his
distance and displacement?
4) One day Mr. Corelli travels from Ngau Chi Wan station to Yau Ma Tei then Hung Hom then Shek Kip Mei
then Lai Chi Kok then Tsim Sha Tsui then Central and then back to Ngau Chi Wan. What is his
displacement? (hint: this question is nowhere near as difficult as it looks)
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Velocity Questions
If two cars, both moving at 60 km/h, passed each other travelling in opposite directions on a road, they
would have the same speed but different velocities.
The velocity of one car might be 60 km/h due east and the velocity of the other 60 km/h due west.
60 km/h east
60 km/h west
North
Velocity is completely defined by a size and direction.
1. What is the difference between velocity and speed?......................................................................................
...............................................................................................................................................................................
...............................................................................................................................................................................
2. A model train travels at a constant speed of 3 cm/s around its track. Write down the train's velocity at
points A, B, C and D?
A
D
A=
Hint;
B=
Did you remember
the
C
=
direction?
D=
B
C
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