Day+3


 * Day 3, Introducing Forces.**
 * <>day 4 >> ||

All about balanced forces in various circumstances. The key rule for today was to understand Newton's First Law: "A body continues in a state of rest or of uniform motion unless acted upon by an external, unbalanced force".

We spent some time identifying and labelling force pairs acting on simple physical situations, such as a weight on a spring, a toy car on a slope, a pair of magnets etc. Issues that arose:

Weight, gravity and mass. Identifying the downward force on an object can be problematic for some students. There is some debate on the use of the term 'gravity'. Being strict with our language we should call the downward force 'weight', as the term describing the downward force experienced by a mass due to a gravitational field. In everyday parlance, 'gravity' is often used more loosely to indicate 'the force due to gravity'. 'Weight' or 'gravitational force/force due to gravity/gravity force' are preferable although SATS exams have marked as correct with just 'gravity'. Students need to appreciate that everyday language and physics language differ and that sometimes examinations are fallible but clarity of communication can overcome most obstacles.

Reaction: why don't we fall through the floor? It is hard for students to grasp that the floor exerts an equal and opposite force on them. Understanding the origins and nature of the reaction force helps them to make that intuitive leap. Illustrations from standing on planks and soft surfaces will help. For gifted students, explaining the way that particles interact in terms of charges to produce a reaction force is a valuable extension. Many students (and teachers!) find the 'Action and Reaction' idea - essentially Newton's 3rd Law, quite difficult and counter-intuitive. Remember that it's essentially concerned with interactions. NASA have produced some useful materials on the topic on this part of the website [|'From stargazers to starships'].

Free body diagrams: Again, here the key is to label and explain diagrams clearly. Key questions: Which forces are acting in opposing pairs? What are they acting on? In which direction does the force act? (In which direction would motion act if it were taken away?) In the case of objects moving under conditions of Newton's 1st Law, consider the effect of changing one force - examples with aircraft, submarines etc can have some counter intuitive outcomes; get students to recognise when they might be 'caught out'. The term 'upthrust' is usually reserved for buoyancy forces, be careful to distinguish from 'lift' - used more generally with aeroplanes, helicopters etc.

Floating and sinking: Although this is a relatively simple practical there are a lot of physics ideas here, thinking about measuring density, weight and volume and looking at correlations between data. Problem solving skills, like working out the apparent change in weight of an object that naturally floats, can be brought in. Consider the teaching and learning aims of a practical activity - students would find it hard to access all of these skils at once. Can you guide students to their own 'Eureka!" moment?

Friction. Another good counter intuitive one, note that there is no surface area component in the equation for frictional force - so questions about the need for wider tyres on high performance cars and why hundreds of pages of interleaved books are harder to pull apart than just a few merit some exploration. The scientific relationship between office chair friction and productivity is explored here.

We finished the day looking briefly at moments and at pressure as what happens to forces when we change the way that they are applied. Some ways of demonstrating phemomena associated with leverage and pressure were discussed.

Some links below to potentially useful resources and some forces diagnostic questions.

SSR article: "Becoming a good forces teacher." (Since the ASE have reorganised their site, you need to be a paid-up member to access this article. Sorry!)