Moving or not moving?
Motion and rest are judged from a reference point.
Runner and scooter
A runner runs along a straight track. Beside the track, a scooter is parked. A friend marks a fixed point O and watches both.
How would you decide which one is moving?
What this lesson covers
The idea
An object is in motion if its position with respect to the reference point changes with time, and at rest if its position does not change with time.
Runner and scooter
A runner runs along a straight track. Beside the track, a scooter is parked. A friend marks a fixed point O and watches both.
How would you decide which one is moving?
- Check whether its position from O changes as time passes
- Check which one is bigger
- Check which one is nearer to the friend
Watch the positions
Press Next step to move the clock from 0 s to 4 s. Look at the distance of each from O.
Does the position change?
Between 0 s and 4 s the runner's distance from O grew from 0 m to 40 m. The scooter was 30 m from O at every reading. So the question to ask each time is the same: does the position with respect to the reference point change with time?
An object is in motion if its position with respect to the reference point changes with time. It is at rest if its position with respect to the reference point does not change with time.
Notes
In motion: the position with respect to the reference point changes with time. At rest: it does not change.
Check yourself
A parked scooter stays 30 m from O at every reading of the clock. The scooter is…
At 0 s a girl is 10 m to the right of O. At 5 s she is 30 m to the right of O. Is she in motion?
An object is at rest when…
- in motion. Its position with respect to O does not change with time, so it is not in motion.
- at rest — correct. Yes!
- in motion, but moving slowly. A very slow object still changes its position with time. The scooter does not change its position at all.
- No, because she stayed on the same track. Staying on the same track does not matter. Her position with respect to O changed, so she is in motion.
- No, because the point O did not move. O is a fixed point. Motion is judged by the change in the position of the girl with respect to O.
- Yes, her position with respect to O changed with time — correct. Yes!
- its position with respect to the reference point does not change with time — correct. Yes!
- it is at the reference point. An object can be at rest at any position, for example 30 m from O. Being at O is not needed.
- its position changes very slowly. Any change of position with time means the object is in motion, even a slow one.