Forces and Pressure
Forces: contact and non-contact
Sort forces by whether they need the objects to touch, read a spring balance, and keep weight and mass apart.
Sonali and Ragini pedal into a headwind, and it is hard work. The road turns rough and it is harder still. Coming back down the hill they stop pedalling and the bicycles pick up speed anyway. Three different things are pushing and pulling on them, and one of them is not touching the bicycles.
What a force is
A force is a push or a pull on an object, arising from that object's interaction with another object — at least two objects have to be involved. Its SI unit is the newton, written with a small n and given the symbol N. A force may set an object moving, change its speed, change its direction, or change its shape.
Worked example
A ball rolling across a flat floor slows down and stops, with nobody touching it. Is a force acting on it?
Start from what a change in speed means: the ball is slowing, so something is acting on it.
The chapter's rule runs both ways. If speed changes, a force is at work — that is the tool for finding hidden forces.
Try it together
A spring balance is marked 0, 1, 2, 3 … in newtons, and between each pair of numbered marks there are five smaller unnumbered marks. Work out the smallest weight it can measure.
Answer with a bare number each time.
1.How much weight lies between the two numbered marks, the one at 0 and the next one, in newtons?
Have a go
Have a go on your own. A boy is holding a bag with a spring balance and the pointer sits three small divisions past 2 N. Each division reads 0.2 N. What does the balance read, in newtons? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.