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1. Collision theory
Collision theory:
Theory that explains how chemical reactions take place and why rates of reaction
alter. For a reaction to occur the reactant particles must collide. Only a certain fraction of the total collisions
cause chemical change; these are called successful collisions.
The successful collisions have sufficient energy (activation energy: a minimum amount of energy
which colliding particles need in order to react with each other.) at the moment of impact to break the
existing bonds and form new bonds, resulting in the products of the reaction.
Increasing the concentration of the reactants and raising the temperature bring about more collisions and
therefore more successful collisions, increasing the rate of reaction.
Changing the Rate of a Reaction.
There are 5 ways to increase the rate of a chemical reaction.
They are all understood in terms of collision theory.
The rate of a chemical reaction may be increased by:
1) Raising the temperature.
2) Increasing the concentration (in solution).
3) Increasing the pressure (in gases).
4) Increasing the surface area of a solid.
5) Use a catalyst.
The opposite of 1, 2, 3 and 4 will decrease the rate of a reaction.
A catalyst (strictly speaking) will change the rate of a reaction.
A catalyst can make a reaction go faster or slower.
In practice a catalyst is mainly used to make a reaction go faster.
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2. Simple Machines
Definitions
-
Any mechanical or electrical device that transmits or modifies energy to perform or assist in the
performance of human tasks.
- A mechanical device that transmits modifies, or changes the directions of force in order to help people do
work.
- A apparatus in which work is done on the machine by applying a force (Effort) at one part which results in
work being done by the machine in overcoming an external force (Weight or Resistance).
The advantage of using a machine is that a small force can be used to overcome a larger resisting
force. Optionally a small movement can be used to cause the machine to generate a larger movement.
There are six basic simple machines:
1.
2.
3.
4.
5.
6.
Incline
Wedge
Lever
Wheel and Axle
Pulley
Screw
The Mechanical Advantage of a machine is the ratio of Force being moved W to the Effort F
Mechanical Advantage = W /F
The Velocity Ratio of a machine is the ratio of the distance moved by the Effort and the distance moved by
the Force being overcome.
Velocity Ratio = Distance moved by Effort/ Distance moved by force
In the ideal frictionless/weightless machine
Velocity Ratio = Mechanical Advantage
The efficiency of a simple machine
Efficiency = Work done by the machine / Work supplied to machine
The only parameter that can be determined from the machines dimensions is the velocity ratio.
Machines generally follow the linear rule.
F = a + b. W
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3. Kind of Simple Machines
Incline Plane
One of the simplest of machines is the inclined plane..
The force Force (F) is the effort required to move the Weight (W) up the slope. A movement of the weight
a distance x along the incline will result in a vertical displacement of x Sin(θ)). Assuming that the incline is
frictionless the F required to move the weight up the slope = W Sin(θ)).
Velocity ratio = 1 /Sin(θ)
Wedge
Velocity ratio (Single Wedge) = 1 /Tan(θ)
Velocity ratio (Double Wedge) = 2 /Tan(θ/2)
Lever
Velocity Ratio (First Class Lever) = B /A
Velocity Ratio (Second Class Lever) = (A+B ) /A
Velocity Ratio (Third Class Lever) = A / (A+B )
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Wheel and Axle
Velocity ratio = R / r
Pulley
Simple Pulley..
Differential Pulley..
Velocity ratio = Number of ropes
Velocity ratio = 2.R /(R-r)
Note: A very simple 2-rope pulley is shown. In
Note:Requires use of chain with engaging
practice there can be a number of rotating pulleys
sprockets to prevent slip
on the top and bottom blocks increasing the number
of vertical ropes.
Screw
Velocity ratio = R. 2. / p
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4. Speed and Velocity
In order to understand the difference between speed and velocity, you must first understand the difference
between distance and displacement.
Speed
The rate of change in distance with respect to time. Since speed is built from distance, a scalar quantity,
then speed is also a scalar quantity. This means it carries no direction information with it.
(the bar indicates average and the delta means change)
Velocity
The rate of change in displacement with respect to time. Since displacement is a vector quantity, then
velocity is also a vector quantity. It has both magnitude and direction.
This formula is considered the definition formula for average velocity.
Both speed and velocity are typically measured in units of miles per hour, kilometers per hour (Km/hr), or
meters per second (m/s).
Acceleration
Acceleration is defined as the rate of change of velocity. Acceleration is inherently a vector quantity, and an
object will have non-zero acceleration if its speed and/or direction are changing. The average acceleration is
given by
Average acceleration is the change in velocity (Δv) divided by the change in time (Δt). Instantaneous
acceleration is the acceleration at a specific point in time which is for a very short interval of time as Δt
approaches zero.
The units for acceleration can be implied from the definition to be meters/second divided by seconds, usually
written m/s2.
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5. Density
Density is defined as an object mass per unit volume. Mass is a property.
Mass and weight are two common often misused and misunderstood terms in mechanics and fluid
mechanics.
The fundamental relation between the mass and the weight is defined by Newton's Second Law and can be
expressed as
F=ma
(1)
where
F = force (N)
m = mass (kg)
a = acceleration (m/s2)
Mass
Mass is a measure of the amount of material in an object, being directly related to the number and type of
atoms present in the object. Mass does not change with a body's position, movement or alteration of its
shape, unless material is added or removed.
The mass is a fundamental property of an object, a numerical measure of its inertia and a fundamental
measure of the amount of matter in the object.
Weight
Weight is the gravitational force acting on a body mass. Transforming Newton's Second Law related to the
weight as a force due to gravity can be expressed as
W=mg
(2)
where
W = weight (N)
m = mass (kg)
g = acceleration of gravity (m/s2)
The density can be expressed as
ρ = m / V = 1 / vg
(1)
where
ρ = density (kg/m3)
m = mass (kg)
V = volume (m3)
vg = specific volume (m3/kg)
The SI units for density are kg/m3.
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