Day+5,+Newton's+Laws+and+forces.


 * <>day 6 >> ||

You should be able to interpret situations in which Newton's First Law applies, identifying balanced forces in moving systems. Be aware that the force may not act in the direction of motion - this is counterintuitive for many students. Consider the example of orbiting bodies - the motion is circular (the instantaneous direction of travel actually tangential to the orbit) but the force acting is gravity - acting towards the centre of the planet. Thus the acceleration is also towards the centre of the planet. Hence it is correct to describe the Moon as accelerating towards the earth. Many balanced force scenarios involve a body's terminal velocity. This tends to be associated with falling objects although it is equally correct to consider terminal velocity in terms of sinking objects or, for example, cyclists travelling as fast as they can pedal. Terminal velocity is a good subject for investigation-based work, bringing in a lot of variables that are relatively easy to identify and control and allowing bright students to be extended with considerations of viscosity - perhaps even researching aspects of Stokes' Law. (More information about Stokes' Law here - a bit heavy but may serve as a good introduction to //dimensional analysis// - which is a handy thing to know about for A level) Fluid dynamics is a massive field - anyone going into aerodynamic/hydrodynamic design for the automotive or aviation industries will need to get into it.

Newton's second law relates the net force acting on a body, its mass and acceleration. Although the definition is not hugely straightforward, the calculations done from it usually are. This is another good area to derive experimentally but consider the purpose of the practical. The idea is to enable learners to see the relationship between the variables so it is a good idea to use technology appropriately to minimise the date processing done by students. If you have light gates that can simply show acceleration, this makes the teaching points far more accessible to students.

Questions on this formula tend to be straightforward, involving only constant force and linear acceleration. Some questions attempt to catch students out by mixing up units - especially putting mass in tonnes.

Practical activities on the day: Measuring force, mass and acceleration. Use of a variety of vehicles and surfaces from dynamics trolleys on a friction-compensated runway to hover pucks across the table top. Use of various methods to measure acceleration. Key points to think about: What is the purpose of the practical? If it is to reinforce the relationship F=ma then the practical should be free of complications and as easy as possible for the students to arrive at the numbers; set up as much of the practical as is possible in advance, use light gates or similar to read acceleration directly. If the purpose of the practical is to reinforce students' ability to deal with and process data then give them more freedom about what data is collected. Uniform acceleration can be worked out from rearranging a = (v-v0)/t; much easier when v0 is zero. If the purpose of the activity is to get students to think about experimental design, leave them to decide on how the accelerating force should be varied and measured. One way in which we did that was to flick the hover pucks with a rubber band tied between two retort stands, leading to the question of how students could ascertain whether the extension/force relationship in that particular configuration was linear: both the mathematical solution (resolving forces) and the experimental one (calibrating with a newton meter) are interesting to consider.

Measuring terminal velocity. This is an excellent activity for open-ended investigational work - you can have a 'target' activity like egg parachutes or a more investigation-based activity like factors affecting terminal velocity. We used wallpaper paste as a cheap source of variable-viscosity medium. You should use fungicide free paste (although remember that wallpaper paste is used by people all the time without ill effects). Instant mashed potato can also be used to make a starchy gel and at low viscosities is still sufficiently translucent to see a sinking object.

Calculations: Some time was spent looking at and devising calculation-based problems. This is a useful activity for you and for students. Students become more familiar and confident with calculations from approaching them from both sides.