BPhO challenge toolkit

They are balanced

提示Steady speed means no change in motion — so what must the resultant force be?

为什么Balanced forces do not mean "not moving"; they mean "not changing". A steady speed in a straight line is exactly what you get when the upward drag equals the downward weight, so the resultant is zero. Only an unbalanced force can speed something up, slow it down or bend its path — which is why a challenge question that says "constant velocity" is quietly telling you the forces cancel.

time

提示Think about what the stopwatch is for in a 100 m race.

为什么The unit tells you the formula: metres (a distance) per second (a time). Sanity-check any speed answer against numbers you know — a brisk walk is about 1.5 m/s, a sprinter about 10 m/s, a car on the motorway about 30 m/s, and sound in air about 340 m/s. In a quiz with a minute a question, that quick estimate is what catches a slipped decimal point or an upside-down division.

Weight

提示The one of the pair that changes when you travel to the Moon.

为什么Weight is a force, so it takes the unit of force, the newton. Mass is the amount of matter and is measured in kilograms; it does not change when you travel. Take a 60 kg astronaut to the Moon and they are still 60 kg, but the pull on them drops from about 600 N to about 100 N because the Moon pulls roughly one sixth as hard. Bathroom scales measure a push and label it in kilograms — a shortcut that only works on Earth.

10

提示A round number; the precise value is a whisker under it.

为什么Weight = mass × g. On Earth g is about 9.8 N/kg, but challenge questions nearly always let you round to 10. Always write the unit: 3 kg is a mass, 30 N is a force, and a bare 30 is only half an answer.

The gravitational potential store

提示It has been lifted up; nothing is moving yet.

为什么Energy is described by stores and transfers. Lifting the ball fills its gravitational potential store; releasing it transfers energy into the kinetic store as it speeds up, and into the thermal store of the slide and the air through friction. The total never changes — it only moves between stores. The other names to have ready are elastic potential, chemical, thermal, magnetic, electrostatic and nuclear.

1000

提示A kilometre is this many metres.

为什么So 2 kW is 2000 W, and a watt is a joule every second — the kettle transfers 2000 J of energy each second it is on. The prefixes are the same in every unit: kilo- ×1000, mega- ×1 000 000, centi- ÷100, milli- ÷1000. Challenge questions love to hand you a mixture of km, m and cm in one sentence; converting everything to one unit before calculating is the step that saves the mark.

ammeter

提示It sits in series with the component, and its name says what it counts.

为什么An ammeter reads current in amperes (A) and must be placed in series, so the current passes through it. A voltmeter reads potential difference in volts (V) and is connected in parallel, across the two ends of the component. In a series loop the current is the same at every point, so it does not matter where in the loop the ammeter goes — the reading is identical.

A parallel circuit

提示Think of the lights in a house — one bulb blowing does not black out the lot.

为什么In a series circuit there is a single loop, the current is the same everywhere, and one break stops everything. In a parallel circuit each branch has its own path back to the cell, so each branch gets the full voltage of the supply and a break in one branch leaves the others working. Two identical bulbs in parallel are each as bright as a single bulb on its own; in series they share the voltage and are dimmer.

Nothing

提示A vibration has to be passed on from particle to particle — so remove the particles.

为什么Sound is a vibration carried by particles bumping into their neighbours, so it needs a medium: a solid, a liquid or a gas. Pump the air out and there is nothing left to carry it, though you can still see the hammer striking — light needs no medium at all. That is why space is silent, and why sound travels faster through water and faster still through steel, where the particles are held together more strongly and pass the vibration on more quickly.

340

提示Roughly a million times slower than light — count the seconds from flash to bang and divide by three for kilometres.

为什么The speed of sound in air is about 340 m/s — UK textbooks quote 330, 340 or 343 depending on the temperature assumed, and a quiz will accept any of them or tell you which to use. Light travels at about 300 000 km/s, close enough to instant over any distance on Earth, so the flash arrives at once and the sound trails behind at roughly a third of a kilometre per second. The same fact runs echo questions: a clap that returns after 2 s has travelled to the cliff and back, so the cliff is about 340 m away, not 680.

The wood

提示Same volume, different mass. Which block squeezes fewer grams into each cubic centimetre?

为什么Density = mass ÷ volume, and water is the yardstick at 1 g/cm³ (1000 kg/m³). Wood at roughly 0.7 g/cm³ floats; iron at about 7.9 g/cm³ sinks. Size is a distraction — a huge log floats and a tiny nail sinks. What matters is whether the object is less dense or more dense than the water it pushes aside, because that displaced water is what provides the upthrust holding it up.

force; area; pascal

提示The same push over a smaller patch is a bigger squeeze; the unit honours a French scientist.

为什么One pascal (Pa) is one newton spread over one square metre, so it can also be written N/m². Halving the area doubles the pressure, which is why a person on one foot exerts twice the pressure on the floor as on two, why a drawing pin goes in point-first, and why snowshoes stop you sinking. The unit is named after Blaise Pascal.