Other physic Book Note On Things Learnt Or Not

Motion: 

the modern study of motion is divisible into two distinct parts:

kinematics and dynamics.

kinematics is the study of motion without regard to its causes. dynamics investigates the causes of motion, that is, it looks at forces. 

Describing motion:

terms used when describing motion are: distance, speed, velocity and acceleration.

the simplest motion is linear or rectilinear motion, that is, motion in a straight line. the simplest of these is motion with constant speed and the next simplest is motion with constant acceleration. complex motions include circular motion and projectile motion.

Motion occurs when an object changes its position relative to other objects or some coordinate system.

Quantitative description of motion:

science is quantitative, that is, it relies heavily on measurement for its description. for motion, the most important quantitative description is the speed of an object.

SPEED

the speed of the object is the rate at which the object changes its position.

speed is the time rate of change of distance.

there are two types of speed, average and instantaneous.

Average speed:

the average speed of an object is defined as: average speed = distance / time

units: the common everyday unit of speed is kilometres per hour, whereas the scientific unit, is metre per second. it follows that. 

1 km / h = 1000 m / 3600 (hour to seconds).

instantaneous speed:

speed at any particular instant is called instantaneous speed.

v = triangle s / triangle t

where the triangle, delta, means a change in. the instantaneous speed is equal to the change in distance divided by the corresponding small change in time. 

constant speed:

an object such as a car is moving with constant speed when it travels equal distances in equal intervals of time. in the special case of motion with constant speed with constant speed, the average speed and the instantaneous speed are numerically the same, that is, they have the same value. 

Speedometer:

a cars speedometer measures the speed of a car at each instant, that is, it measures the instantaneous speed (to a good approximation).

Measuring speed:

to calculate speed of an object we need to measure distance and time.

to measure distance and time to calculate speed might use the following:

a ruler and stop watch.

a tickertimer.

electronic and/or computer timing.

a blinky/strobe utilising multiflash photography.

Ruler and Stopwatch

these two devices re useful when we are dealing with relatively long time intervals (anything more than a few seconds). for example, we might use them to determine the average velocity of a runner at a school sports carnival bytes method. it would however, be difficult to use this method to measure the runners, instantaneous speed. this is where the following methods can be used:

ticker timer: 

this device consists of a magnet and a hammer arrangement that results in the hammer marking paper ticker tape at regular time intervals as the object pulls the paper through the timer. the timers generally operate at the electricity mains frequency of 50 Hx, that is the hammer strikes the paper 50 times per second, leaving a mark on it. it follows that the time interval between strikes is 1/50th of a second. 

the distance between successive dots indicate the speed of the tape and hence the object to which it is attached. the closer the dots, the slower the object was moving, the further the dots are apart the faster the object,

Momentum

a truck requires a large force to set it in motion when it has a full load than when it is empty. likewise, far more powerful brakes are needed to stop a train then a pushbike moving with the same velocity. when two bodies, a heavy one and a light one, are acted upon by the same force for the same tiem, the light body builds up a greater velocity than the heavy one. the heavy vehicle is said to have a greater amount of momentum than the lighter one.
if we take newtons second law f - ma and replace the acceleration with its definition as the time rate of change in velocity, a = v / t.
f = mv / t
the force is now defined as the change in mv with time.
the produce of mass and velocity is called momentum and we give it the symbol p.
momentum = mass x velocity.
p = m v
where p = momentum measured in kg m s -1
m = mass measured in kg
v = velocity measured in ms to negative 1 square.
if we assume that f is zero, then the change in momentum p must also be zero. thus if there are no external forces acting on an object, the momentum of the object is constant.
momentum is a vector measurement. it is an indication of how hard it will be to stop an object.
the momentum of any object will never change if left alone.

Friction & The Difference Between Mass and Weight

The Difference Between Mass and Weight:
mass is the quantity of matter that makes up an object. it is measured in kilograms.
the weight of a body is the force it normally exerts on anything which supports it. a body exerts this force owing to the fact that it is itself attracted towards the earth by the force of gravity.
weight = mass x acceleration due to gravity
f     = mg
  w
a 1 kg object on earth weighs 1 x 9.8 = 9.8 N. the same object on the moon, where the acceleration due to gravity is 1 / 6 that of the earth, weights 1 x 9.8 / 6 = 1.63 N. the object has the same mass in both places, but the weight is different, due to the differences of the gravitational pull of the two planets.

Friction:
friction is a contact force that opposes motion. friction slows a moving object down. friction is also the force that stops stationary objects from moving. it plays an important role in our daily loves. walking would be impossible if there were no friction between the ground and the soles of our shoes. without friction, brakes would not work.
friction can be a nuisance, because when two surfaces rub together they convert kinetic energy into heat energy. when an object becomes hot, it expands and takes up more space. the inside of a car engine becomes very hot. it is necessary to reduce the heat generated, and consequently reduce the expansion of the engine components. we add oil to an engine to do this. a layer of oil between two surfaces acts as a cushion, reducing the friction between two surfaces rubbing together. air resistance is another example of friction. the greater the velocity of an object moving through the air, the greater the amount of air being pushed out of the way by the object. air resistance depends on the shape and the speed of the object. a truck with a large surface exposed to the air has to move a greater amount of air, therefore a greater force us needed to move at it at the same speed as a smaller object, such as a car. modern cars are designed to cut through the air with the least resistance, therefore they have small surfaces exposed to the air.

Newton's Law's Of Motion

Newton's First Law of Motion: the Law of Inertia
An object will continue in a state of rest, or uniform motion in a straight line at constant velocity, unless an unbalanced external force causes it to change the state.
inertia is a property of any object. it involves both the amount of motion (velocity) and the mass content ( mass of an object).
the law of inertia is simply the statement that any object resists any change in its state of motion or state of rest if not moving.
the degree of resistance an object offers to a change in motion is directly proportional to the mass of the object. it is much easier to move a 1 kg mass than it is to move a 100 kg mass. this is because 100 kg has more inertia. to move each vehicle, you have to overcome its inertia. because the 'motorcycle' has less mass it has less inertia.

Newton's Second Law Of Motion:
an unbalanced external force will change an objects state of motion by producing an acceleration. the force is equal to the product of the mass and acceleration of the object.
force (net) = mass x acceleration.
f net = ma
m = mass in kilograms (kg)
a = acceleration in metres per second
fnet = net (unbalanced) fore, which produces the acceleration in newtons.


Newtons Third Law of Motion:
for every action (force) of an object, there is an equal and opposite reaction by the object upon the agent (for every action done by an object, the object that had just been placed with this action will exert an equal and opposite reaction back open the object). when you walk, your foot pushes on the ground while the ground pushes upward on your foot, the two forces being equal and opposite.
forces always come in pairs. important points to remember in relation to netwon's third law are that the two forces, the action and reaction force, are both equal and opposite, and they are acting on different objects.