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Physics 2010 Nagging

Posted on October 31, 2010 at 2:28 AM Comments comments (1)

Dear all, i can't emphasis the importance of this. READ CAREFULLY.

Otherwise do remember the following pointers.

 

• Everything in 3sf. Don't waste your brain cells struggling the number of sf, just stick to 3. What happens if its exact value. E.g. 1.23N x 3.21m = 3.9483 Nm. So write that down its ok. But after this step, write this again. =3.95 Nm (3 s.f.)

• What to bring for your exams.

o Calculator that works

o Compass and projector (those pesky resultant force questions)

 

• Speed time graph questions when you are stucked, don’t forget for every section of your speed time graph label AVUTD.

 

• When does an object experience terminal velocity when it is free falling? Keep points include the weight of the object is the same as the air resistance of the object. When the object moves faster, the air resistance increases. When air resistance is the same as its weight, resultant force acting on the object is zero (this is the main thing i hope you will remember, any resultant force question, you have to phrase it as RF acting on object), therefore acceleration of the object is zero, object moves at constant speed.

 

• Angles always 1d.p.

 

• Friction depends on mass of object, surface texture and speed of object. Mass increase, friction increases. Speed increases, friction increases.

 

• How to calculate friction – a few formulae to calculate them

o Work done against friction = frictional force x distance moved

o Frictional force = mass x deceleration

o Resultant force = applied force - friction

 

• For pure physics people, becareful of the upthrust question when an object move upwards. When an object moves upwards, resultant force = upthrust – weight.

 

• There are questions that ask you whether an object will topple off the slope or not. E.g. in the 2009 paper the lorry question. You need to say something like the line of action of its weight is acting within the area of contact, The weight creates a moment that pushes the lorry onto the slope keeping it stable. Otherwise if it topples, say the line of action of its weight is outside the area of contact, therefore the weight creates a moment that topples the lorry.

 

• For those kind of slope questions, consider lifting an object of mass 10kg up this slope

 

• If question asks you about work done to lift the object up the answer is simply PE = mgh. In a way 10 x 10 x 3 = 300J. That’s work done to lift it up and i use 3m because the question gives us the mass.

 

• However if the question asks us for the pulling force to lift the object up the slope? Idea is that you still need 300J to pull it up. Therefore the pulling force is based on 5m length, work done = force x distance. 300 = f x 5 = 60N. So to pull the object

up the slope, a force of 60N is needed to pull the object up the slope over a distance of 5m. Supposed the pulling force is 80N instead of the required 60N. Work done due to the 80N force is 80 x 5 = 400J. You notice that 100J extra is needed instead of the 300J. That extra 100J is work done to overcome friction.

 

• Convert g/cm3 into kg/m3 simply x1000

 

• Moment remember to look out for PERPENDICULAR distance to pivot, not perpendicular to your view.

 

• Nervous, Nervous

 

• For pure Literature people i pity you, please endure.

 

• For light questions, if your light ray is moving from dense to less dense medium, never assume refraction occurs. Always check if the angle of incidence in the denser medium is more than critical angle. If it’s more than critical angle, it will undergo total internal reflection.

 

• Fat lens have short focal length, skinny lens have longer focal length.

 

• Magnification = image height/object height

 

• Magnification = image distance from lens / object distance from lens

 

• Speed of light 3 x 108m/s is defined as speed of light in VACUUM not air.

 

• CHECK THAT YOUR CALCULATOR IS NOT IN RADIAN MODE!!!!!

 

• EM waves, you need to memorized position of the waves. Note that high wavelength EM waves like radio,

 

• Sound moves fastest in solid, slowest in air, since sound energy is transmitted by vibration of particles, the closer they

are together, the faster it is transmitted.

 

• When doing sound calculations, make sure you account for distance x 2 or time x 2 since it’s echo thing.

 

• Stupid formulae

o Power = force x velocity

o V = J/C (Victoria jc), EMF = WD (energy)/Charge

 

• When doing questions on brightness of light bulbs, don’t worry if you have poor imagination, just assigned values to it. EMF let it be 12V, every resistors 2ohms, It should give you nice numbers.

 

• Calculating how much electricity used, units of electricity = power in kW x Time in hours

 

• Why must the fuse be in the live wire? When the fuse is in the live wire, it breaks the high potential in the live wire preventing electrical shocks to user. If it is in the neutral wire, although the circuit is broken, the live wire is still at high potential, this causes danger to the user if an electrical fault is present.

 

• Don’t mixed up fuses and earth wire. Fuses prevent high current from damaging the equipment. When the current is higher than the fuse rating, the fuse will melt and breaks the circuit. For earth wire, it’s a safety feature to protect user in the event that the metal casing is live. If the casing becomes live due to the electrical fault that causes leakages of current to the metal casing. The earth wire will lead the current to the earth thus protecting user from electrical shocks. Hence do not blame SHORT CIRCUIT FOR EVERY OF YOUR PROBLEM, although most of the time sudden increase in current is due to short circuiting.

 

• Thin wires – Pro- cuts cost since less material is needed.

 

• Thin wires – Con – thin wire has high resistance, thus producing a lot of thermal energy lost due to heating effect of the wire. This is because power loss due to heating effect of wire can be calculated by P = I2R. Since R is higher for thin wire, the heat loss will also be greater.

 

• I against V, gradient is 1/R, while V against I, gradient is R.

 

• Electrical field lines – out of positive charge, going into negative charge ALWAYS

 

• Magnetic field lines – out of north, going into south

 

• For Fleming’s Left Hand Rule, make sure you write the following template. The interaction between the magnetic field and the current produces a force. Using FLHR, the force created is ___(in certain direct_____.

 

• AS OF THIS POINT, Kenneath & Edwin, Melvin, Jet, Grace, that’s it for you. If your name is not these 5, carry on reading.

 

• Positive static charges NEVER MOVED!!!!! Whenever you talk about induction of static always talk about the charges in your sphere, comb, paper, whatever, before you say electrons are attracted or repelled.

 

• What are the possible reasons why a transformer is not ideal? Standard answers will be heating effect of wires, eddy current produced on the soft iron core,

• Why transformer needs an a/c source? Remember its that changes in magnetic flux needed for a continued induction of EMF. Full story in template.

The Confusion of Vectors and Scalars

Posted on May 31, 2010 at 11:46 PM Comments comments (0)

One of the more commonly asked question is the classification of vector and scalar quantities.

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Basically we know that vector quantities has both magnitude (value like 2m/s, 400m etc) and direction (which means there is a possible of positive or negative figures like -5.2m/s).

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Fortunately, an easy rule can help us to remember them.

Most of us can remember the 3 usual suspects

DISPLACEMENT, VELOCITY AND ACCELERATION.

The last member of this select group is FORCE.

Basically anything that has the unit of NEWTON, we can safely classify it under vector quatity. For example, friction, weight, pulling force, pushing force, contact force etc. The list can go on and on. You get the drift.

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Which brings us to scalar, quatities which has magnitude and no direction. Since we are able to classify vectors, simply label the other components as scalar. Its that ez!

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