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Mechanics
Force
of Gravity, Weight, Friction, Speed, Velocity, Acceleration, Newton's Laws of
Motion
Vectors, Turning Forces, Work, Energy, Power, Machines, Density, Relative
Density
Pressure in Liquids and Gases,
Atmospheric Pressure, Archimedes' Principle, Heat and Energy,....more.
Math
Help! Name
your topic, any topic and state your concern.
Get
some answers.
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Featured
Topics
Linear
Motion - Speed, Velocity, Acceleration
Linear Motion is
motion in a straight line.
Speed is defined as
the rate at which distance changes with time. In other words, Speed
= Distance/Time
In many cases a motorist traveling over any distance will face varying
traffic conditions, either to slow down, stop or speed up. Whatever there
are, by the end of the
journey, he would have done an average speed.
Average Speed
= Total Distance/Total time taken
Traveling from point A to point B, a distance of 144 miles in 2 hrs.
would give an average speed of 144 = 72 miles/hr.
2
Velocity is
speed in a specified direction. It is a vector quantity while speed is a scalar
quantity.
Acceleration is
the rate of change of velocity with time. Let's say a motorist comes to a stop
at point B from a speed of 152 meters per sec. (m/s) in 20 sec. The final
velocity then would be 0 m/s. The initial velocity is 152 m/s.
Change in velocity = final velocity - initial velocity
= 0 - 152 = -152 m/s.
The rate of change of the velocity would be -152/20. That is, -7.6 m/s2.
The minus sign tells the car is decelerating.
Equations of Motion
A car moving from point A to point B will move in a given
time t.
The velocity at which it leaves point A is the initial velocity u,
while that of reaching point B is the final velocity v.
The distance between A and B is given as s
and the acceleration, a.
There
are three main equations of motion.
1. v
= u + at
2. s
= ut + ½at2
3. v2
= u2
+ 2as
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Example 1
A
car travels at a uniform velocity of 20 m/s for 5s. The brakes are then
applied and the car comes to rest with uniform retardation in a further
8s. How far does the car travel after the brakes are applied?
Initial Velocity = u = 20 m/s
Time traveled after brakes = t = 8s
Distance traveled after brakes = s
Applying
v = u + at (where
a is the retardation)
0 = 20 + 8a
8a = -20
a = -20/8
= -2.5 m/s2 (note
the minus sign for
retardation)
Applying
s = ut + ½at2
(for the distance traveled after
braking)
s = (20)(8) + ½(-2.5)(8)2
= 160 - (64)(5)/4
= 160 - 80
= 80 m
Example 2
A
motorist, traveling at 90 km/h, applies his brakes and comes to rest
with uniform retardation in 20s. Calculate the retardation in m/s2.
Initial Velocity =
u = 90 km/h = (90 X 1000)/3600 m/s = 25
Final
Velocity = v = 0
Time taken = t = 20s
Applying
v = u + at
0 = 25 + 20a
- 20a = 25
a = -25/20
= -1.25 m/s2
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Newton's Laws of Motion
1.
A
body continues in it's state of rest or of uniform motion in a straight line,
unless compelled by some external force to act otherwise. 2.
The rate of change of momentum of a body is proportional to the applied force
and takes place in the direction in which the force acts. 3.
To every action there is an equal and opposite reaction.
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Reference: Abbott, A. F. "Speed, Velocity, and Acceleration." Ordinary Level Physics, 4th ed. Heineman Educational Books Ltd. 1984
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