Understanding ‘Force’

Understanding ‘Force’ This document can be freely copied and amended if used for educational purposes. It must not be used for commercial gain. The author(s) and web source must be acknowledged whether used as it stands or whether adapted in any way. Download K4.2_1.0a ‘Forces lead lecture’ Authored by Keith Ross, University of Gloucestershire accessed from http://www.ase.org.uk/sci-tutors/ date created February 2005 Forces lead lecture

Forces The word force is used widely in everyday English: • police force, • 'Dad forced me to do it', • 'May the force be with you', • 'Don’t force it — it might break', • G-force. Forces lead lecture

Aristotle andNewton • 1600s Galileo and Newton gave us a new picture, revolutionising the older ideas derived from the Greek philosopher Aristotle. • What is interesting is that this same revolution is needed in each one of us. • The ideas of Aristotle are descriptive common sense, and we need to undergo a ‘paradigm shift’, just as Newton did, to come to our present scientific understanding. Forces lead lecture

1) What can a force do? makes something start moving makes something stop moving makes something that's already moving go faster makes something that's already moving go slower causes a turning movement causes something to change shape • A push • A pull • A force at a distance Forces lead lecture

You fire a pebble using a catapult and it hits a tin can. • Pull the elastic — causes elastic to change shape. • Let it go applying a push to pebble — makes pebble start moving. • Pebble make a curved flight path – causes a change in direction • Pebble hits can: can pushes pebble — makes it stop moving. • Pebble pushes can — makes it change shape. Forces lead lecture

You take your loaded supermarket trolley round a corner and stop at the check-out. • You change direction whilst running on a football pitch by digging your boot studs into the ground. • You throw a ball in the air and let it fall to the ground where it bounces. • A Catherine wheel firework operates. • You kneed dough, or model plasticine. • A golf ball makes a curved flight through the air. Forces lead lecture

2) Force-at-a-distance Is the wind (blow football, wind on leaves) a force-at-a-distance? Is there really any difference between 'contact' forces (eg pushes and pulls) and forces at a distance (eg gravity and magnetism)? Forces lead lecture

3) The natural state/direction of motion (if no further human interference takes place) • the air • flames from a fire • a stone that's thrown in the air • water • a cart when the horse stops pulling it • the sun & moon Air Fire Earth Water Forces lead lecture

4) A car travels at a constant 60 mph, along a flat road. • There is a forward force from the engine, and a backward force from wind resistance, friction etc. • Is the forward force greater than, equal to or less than the combined backward forces? Forces lead lecture

At the top Push from hand Gravity Half-way up Half-way down Gravity Gravity Gravity A ball has been thrown up mark any forces (neglect air resistance). Science Issues - Atmosphere Forces lead lecture

6a) When you release objects from a high building on Earth, do • do the objects drop at aconstant speed, with heavy objects (like a stone or person) dropping faster than lighter objects (like a spider or feather)? • Or do the objects gradually pick up speed and fall faster and faster, but with everything, light or heavy, falling together? Forces lead lecture

6b) What happens if you are on the moon and release the same objects? • do the objects drop at aconstant speed, with heavy objects (like a stone or person) dropping faster than lighter objects (like a spider or feather)? • Or do the objects gradually pick up speed and fall faster and faster, but with everything, light or heavy, falling together? Forces lead lecture

Terminal velocity (not to scale) Forces lead lecture

Children’s notion of up and down From Children’s Ideas in Science Ch 9 Forces lead lecture

The children are standing on the Earth. They are all holding stones. They let go of them. Draw lines to show where they all go. Forces lead lecture

“The Earth is round” From ‘Children’s Ideas in Science’ p182 Forces lead lecture

Concept cartoons • http://www.conceptcartoons.com/index_flash.html Forces lead lecture

Foundation/Key stage one • Children may say things move because of the wheels, engine or legs, eg 'Its wheels make it roll down the slope'. • They may say that things fall because you dropped them. • They may say moving objects get tired, or run out of petrol, to explain why they stop. • They may say things float because the water is shallow, or the object is ‘light’ (but this might be correct, if ‘light’ means low density). • They may say that the faster something moves the more force it has (here the word ‘force’ is more like the scientific word ‘momentum’). Forces lead lecture

Key stage 2 • gravity as a force at a distance. • the idea of friction to explain why toy cars slow down • The material is important in deciding if something will float — things that are light for their size will float • objects at rest are subjected to balanced forces • unbalanced forces will lead to objects slowing down, speeding up or changing direction. • An object moving at constant speed in a constant direction also has balanced forces (Key Stage 3). This problem avoided at KS2 where it says ‘that forces acting on an object can balance, and that when this happens, an object at rest stays still'. Forces lead lecture

Understanding ‘Force’