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30 Physics
Dynamics Notes
Kinematics is the science that describes the motion of objects. The "how of
motion" (dynamics) explains why objects change their motion.
The physical phenomena responsible for changing an object's motion is
FORCE.
A force is a push or pull. It's symbol is F and the unit for force is the
Newton(N).
A 1 Newton force is that force required to change the velocity of a 1.0 kg
object by 1.0 m/s in 1.0 seconds. As such there is an equivalent unit for
force:
1N 
1kgm
s2
or
1N  1kgms2
Newton's Law's of Motion
1. An object maintains constant velocity unless acted upon by an
unbalanced (or non-zero net) force. This is sometimes called Galileo's
law of inertia. One property of matter is its inertia....it's tendancy to
remain at constant velocity. Matter is inert to chage as far as motion
(velocity) is concerned.
2. The acceleration of an object is directly proportional to the net force
acting on it
aF
and inversely proportional to its mass.
a
1
m
Together we get Newton's second law.
F  ma
3. For every action (force) that object A exerts on object B, object B
exerts an equal force on A but in the opposite direction.
Weight & Mass
Weight and Mass are NOT THE SAME!!
Mass is a measure of the amount of matter in an object (measured in
kilograms) and weight is the force of gravity pulling down on an object
(measured in Newtons). So why do so many people confuse the two
and/or not differentiate the two?
It just so happens that the more mass an object has the more it weighs
and in fact mass and weight are proportional. So if an objects mass is
doubled so is its weight....but they are NOT THE SAME!!
Gravitational force depends on the mass of the two objects involved and
the distance separating their centres. As long as one of the objects is the
earth (which is typically the case when we are discussing the weight of
an object) then this force of gravity (weight) is simplified to:
Fg  mg
where
Fg is the force of gravity (weight)
m is the mass of the object and
g is the gravitational field strength of (in this case) earth and is
equally to 9.80 Nkg-1
Consider this. If you were in a spaceship in deep space and the space
ship accelerated at a rate of 9.8 ms-2, what would it feel like? What if
you were accelerating at 4.9 ms-2, or 19.6 ms-2?