key ideas and revision Physics

Key Concepts of Physics by Chapters 1. Physical Quantities, Units and Measurement • Memorize 7 base SI unites, prefixes • orders of magnitude e.g. Diameter of atom (10-10 m), Diameter of Earth (6 x 106 m) Refer to textbook • Use appropriate measuring apparatus (vernier calipers, micrometer screw gauge) • Pendulum experiment, measurement of diameters of circular objects • Vectors diagrams (vectors, resultant vectors), object in equilibrium (closed vector diagram) • Improve accuracy by taking more measurements at different points and finding the average, take timing for more oscillations 2. Kinematics • Describing of graphs (upward trend/downward trend/horizontal line (no change in y axis), straight line/curve) – at rest, constant, increasing, decreasing speed. Constant, increasing, decreasing acceleration and deceleration.) • Use of graphs (actual reading, gradient and area under the graph) • Always sketch the graph to solve problems. Do not use distance = speed X time (only constant speed) • Memorize definitions of distance, displacement, speed, velocity, acceleration, uniform speed/velocity, uniform acceleration and non-uniform acceleration. • Uniform = increase at a constant rate. • Free fall – an object in the air if without resistance will always be accelerating downwards with a value of g. • With air resistance, if moving up, then decreasing deceleration. If moving down, decreasing acceleration. (weight is constant, air resistance changes with speed. When speed is high, air resistance is high. Thus changing the resultant force. Resultant force is decreasing, hence acceleration is decreasing.) • With resistance, a falling body has an initial acceleration of 10m/s2 but experience decreasing acceleration. Weight is constant, but as speed increase, so is air resistance. Hence resultant force is decreasing. Decreasing till its weight = air resistance therefore terminal velocity. An object with a larger mass will have a higher terminal velocity as it experience greater acceleration before reaching terminal velocity. 3. Forces • Consider net force. Net force = 0, (moving at constant speed / at rest) • If there is a net force, then object moves with acceleration / deceleration. • Acceleration may be obtained from calculation or gradient of speed time graph. • Keywords – increasing, decreasing, constant acceleration, constant velocity and how it affects the resultant force. • Factors affecting friction – independent of surface area • Balanced force diagrams, free body diagrams – normal reaction force perpendicular to surface of contact, friction opposes direction of motion of surface in contact. 4. Mass, Weight and Density. • Definition and difference between mass and weight. Do not be ambiguous. E.g mass is constant, etc. • Define gravitational field strength g as gravitational force per unit mass N/kg. • Define inertia. Keywords = reluctance to change it state of rest of motion. • Convert between g/cm3 to kg/m3. • Keywords – mass constant, volume changes, density changes, float or sink. • All objects regardless of mass and size that use the same materials will have the same density. 5. Moment. • Define moment of force – product of the force and the perpendicular distance from the pivot to the line of action of the force. • Extend the force and use your “L” sign to locate the distance.

o When in equilibrium, an object will have balanced forces (e.g. upward forces = downward forces) and cw moment = acw moment. • Uniform ruler => WEIGHT at c.g (usually center for regular objects) • Keyword of c.g – point, appears to act, any orientation of object, consider c.g. as a reference point • If weight is acting a c.g => neutral equilibrium => will remain at position when turn or displaced since no perpendicular distance from pivot to the line of action of the force => no moment => will not turn. • Explain stability using moment cause by the weight of object. Always consider the moment it created using the pivot and the weight. If moment due to weight will enable the object to go back to the original position, then STABLE. • Locating the c.g of object • Bigger base area and/or lower c.g => object need to displace more before the c.g is beyond its base and weight will cause…. Not enough to say weight acts within the base, need to explain it is due to the clockwise/anti clockwise moment due the weight which allows the object to go back to original position. 6. Energy, Work and Power • Take note of how acceleration, deceleration and constant velocity affects KE • Remember to square root when calculating velocity from KE • W.d against gravity = GPE = mgh • W.d against friction = Friction x d • W.d by engine = engine force x d = Gain in GPE + Gain in KE + loss of energy (friction/air resistance/sound/heat) • W.d on object = Gain in GPE + Gain in KE • Power efficiency • Do not say energy loss due to friction!!! Use “energy loss due to work done against friction” • In explaining how conservation of energy is used, e.g. “As the ball moves upwards, GPE increases. As the total energy of the system remains constant, KE decreases, and hence speed decreases.” 7. Pressure • Keywords – Force exerted, Area of contact, Pressure difference thus there is a resultant force exerted on the side which has a higher pressure • Hydraulic systems – use pressure formula on both sides to be equal as the liquid will distribute the pressure evenly. o Liquids are incompressible. o When pressure is applied to a trapped body of liquid, the pressure will be transmitted to all other parts of the liquid and is the same throughout the liquid. o In hydraulic press, small effort applied on a small piston can lift a larger load on a bigger piston. o A1 x d1 = A2 x d2 and F1 x d1 = F2 x d2 Application: car hydraulic disc brake system o when driver steps on brake pedal, pressure on brake is transmitted to large pistons on each side of a large disc on the wheel axle o pistons come into contact with disc, disc slows down due to friction car slows down • H x density x g => H is the depth • Conversion of mmHg to Pa • Higher altitude = lower atmospheric pressure • Factors affecting the height of height difference in liquid of a barometer and manometer o Gravity increase, height decrease o Density of the liquid increase, height decrease o Diameter of tube, no effect • Always remember to add air pressure above liquid pressure unless otherwise stated. 8. Temperature

• Memorize the physical properties that can measure temperature • Responsive, Sensitive and Range • More senstive = smaller range • Fixed points are needed to calibrate the thermometer to define the temperature scale. • Ice point: temperature of pure melting ice at standard atmospheric pressure, assigned a value of 0C • Steam point: temperature at which boiling water changes into steam at standard atmospheric pressure, assigned a value of 100 C • Describe experiment to calibrate ice and steam point • Remember to compensate when doing temperature scale problems. • Cold junction of thermocouple if in hot region will give negative emf • Thermocouple o How it works? Two junctions of two difference metals o Range? o Responsive as metal is a good conductor of heat (do not write sensitive as not stated) 9. Kinetic Model of Matter • Solid – closely packed, regular pattern, vibrate about fixed positions, strong intermolecular forces of attraction. • Liquid – closely packed but slighly further apart than solid, randomly arranged, move and flow pass each other, strong intermolecular forces of attraction. • Gas – very far apart, moving randomly in high speeds, weak intermolecular forces of attraction • Fix Shape? –Regular pattern? • Fix Volume? – forces of attraction to hold particles together? • Can Flow? – fix position? forces of attraction to hold particles together? • Compressible? – space between part