Distance & Displacement (HL IB Physics)

Distance

• Distance is a measure of how far an object travels
• It is a scalar quantity - in other words, the direction is not important

Total running distance

The athletes run a total distance of 300 m

• Consider some athletes running a 300 m race on a 400 m track
• The distance travelled by the athletes is 300 m

Displacement

• Displacement is a measure of how far something is from its starting position, along with its direction
  • In other words, it is the change in position

• It is a vector quantity - it describes both magnitude and direction

Total distance vs total displacement

The athletes run a total distance of 300 m, but end up 100 m from where they started

• Distance is a scalar quantity because...

It describes how far an object has travelled overall, but not the direction it has travelled in

• Displacement is a vector quantity because...

It describes how far an object is from where it started and in what direction

• When a student travels to school, there will probably be a difference in the distance they travel and their displacement
  • The overall distance they travel includes the total lengths of all the roads, including any twists and turns
  • The overall displacement of the student would be a straight line between their home and school, regardless of any obstacles, such as buildings, lakes or motorways, along the way

What is the difference between distance and displacement?

Displacement is a vector while distance is a scalar quantity

• Consider the same 300 m race again
  • The athletes have still run a total distance of 300 m (this is indicated by the arrow in red)
  • However, their displacement at the end of the race is 100 m to the right (this is indicated by the arrow in green)
  • If they ran the full 400 m, their final displacement would be zero