Where exactly is it?
Position needs a reference point, a distance and a direction.
Where is the dog?
Your friend says, "The dog is 40 metres from the gate." You look around, but you still cannot tell where the dog is.
What do you think is missing from the description?
What this lesson covers
The idea
The position of an object at an instant is described by its distance and direction from a fixed reference point.
Where is the dog?
Your friend says, "The dog is 40 metres from the gate." You look around, but you still cannot tell where the dog is.
What do you think is missing from the description?
- The direction from the gate in which the dog is
- The colour of the gate
- The age of the dog
Full or not full?
Tap the box where each description belongs. O is a fixed point on a straight track.
Distance and direction
Saying "40 m from O" fits two different places, one on each side of O. Saying "to the right of O" fits many places. Only when we give both the distance and the direction from the fixed point do we know exactly where the object is.
The position of an object at an instant of time is described by its distance and direction from a fixed reference point.
Notes
The position of an object at an instant is given by its distance and direction from a fixed reference point.
Check yourself
What is a reference point?
A ball is 3 m from a wall. What else do we need to describe the position of the ball?
Two children are both 40 m from O, one on each side of O. Do they have the same position?
- The point where the object stops moving. The reference point is chosen and fixed. It does not depend on where the object stops.
- A fixed point from which the distance and direction of an object are given — correct. Yes!
- A point that moves along with the object. If the point moved, it would not be fixed. The book uses a fixed point as the reference point.
- Its mass. The mass of the ball does not tell us where the ball is.
- Its speed. Speed tells how fast an object moves, not where it is.
- Its direction from the wall — correct. Yes!
- No, they are in different directions from O — correct. Yes!
- Yes, the distance from O is all that matters. A position needs a direction as well as a distance, so the two children are at different places.
- Yes, because both use the same point O. Using the same reference point is right, but the position also depends on the direction from it.