Measurement and Motion
Motion and reference points
Use a reference point to decide whether something is moving, and tell linear, circular and oscillatory motion apart.
Five friends argue about whether the garden is nearer than the school. Each gives a different answer, and each one is right — because each is measuring from their own front door. The moment they agree to measure from the bus stand instead, the argument stops. That agreed fixed point is called a reference point.
Moving, or not?
An object is in motion if its position changes with respect to the reference point as time passes. If its position stays put, it is at rest. Notice that the reference point is part of the answer, not a detail: passengers sitting still in a bus are at rest with respect to the bus and in motion with respect to a building outside. Both statements are true at once.
Three kinds of motion
Motion along a straight line is linear. Motion along a circular path is circular. Motion to and fro about a fixed position is oscillatory. Circular and oscillatory motion both repeat themselves after a fixed interval, which makes them periodic.
Have a play
Each thing in this park shows one of the three kinds of motion.
Tap any part of the picture.
Worked example
Padma is on a bus. A kilometre stone reads 'Delhi 70 km'. Later one reads 'Delhi 60 km'. Is she moving?
Pick out the reference point: the stones are measuring from Delhi, so Delhi is it.
Without naming the fixed point, '70 km' is not yet a position — it is just a number.
Try it together
Now settle a puzzle. Deepa sits in a moving bus and looks at the passenger beside her, who has not stirred.
The answer depends on what you measure from, so take the reference point one at a time.
1.Take the bus itself as the reference point. Is the passenger beside her at rest, or in motion?
Have a go
Have a go on your own. A merry-go-round and a swing both repeat their path after a fixed interval of time. What is the word for motion that does that?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.