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Mr R Gopie PHYSICS
Page 2 of 10
DYNAMICS
Scientists
Aristotle (384-‐ 322 BC) through coarse observations of, and refined speculation about, the motion
of earthly bodies arrived at the conclusion that a force (F) was required to act on a body (of mass
m) to enable it to maintain a uniform velocity (v). In fact his law of motion can be expressed as F α v
(and F α m > F = m x v)
Galileo (1564-‐ 1642) through experimentation and observations challenged Aristotle views and
expressed the view that a force is not required to maintain constant velocity but rather, to cause a
change in velocity ( which is an acceleration)-‐so in effect he postulated that force was required to
cause acceleration ( not constant velocity).
Newton (1642 – 1727) agreed with Galileo’s views and formerly expressed them in three laws
NEWTON`S LAW OF MOTION
First law: Everybody continues in a state of rest or of uniform velocity unless
compelled to do otherwise by some applied resultant force. Example of its
application include the jerk forward (or backward) of a passenger as brakes are
applied to a moving vehicle (or as the vehicle is accelerated forward from rest). Also
the use of seat belts on the passenger to provide a force that changes the state of
motion of the passenger
Second Law: the rate of change in momentum brought about by an applied (resultant)
force is directly proportional to the force and the change in momentum occurs in the
direction of the force. The momentum (p) of a body is defined as the product of its
mass (m) and velocity (v) ,i.e. p = m x v
Mr R Gopie PHYSICS
Page 3 of 10
It is a vector quantity and its unit in kgms-‐1 or Ns.
If a resultant force, F is applied to a mass m, for time, t, causes its velocity to
change from u to v then Initial momentum, Pi = mu and Final momentum Pf = mv.
Therefor change in momentum, ΔP = Pf – Pi = mv – mu
And
Rate of change in momentum = ΔP/t
By Newton`s second law: F α ΔP/t
But rate of change of momentum, ΔP/t = (mv – mu)/t
= m (v-‐u)/t
= ma
So F α ma then F = kma. However since the unit of F, i.e. the Newton is
defined as that force which causes a mass of 1kg to have an acceleration of 1ms-‐2 (i.e.
since F = 1N when m = 1kg and a = 1ms-‐2) then k = 1.
So arising from newton’s second law (and the definition of the S.I. unit of force) is
the equation
F = m x a (and this is an alternative way of expressing newton’s second law of
motion)
Also based on the definition of the newton together with newton’s second law, the
(resultant) force is the rate of change of momentum, i.e.
F = ΔP/t
Mr R Gopie PHYSICS
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= mv – mu /t = m a
Examples of its application include: i) bending the knees upon landing on the
legs after jumping from a height-‐ so as to increase and so decrease force ii) using
seatbelts that are somewhat elastic – not enough to allow contact with a solid
obstacle such as the windscreen but enough to increase time and so decrease force
F. also the use of air bags – to more effectively do the same as seat belts, but with a
lower possibility of injury to the user.
Third Law: if body A exerts a force on body B (the action) then body B exerts an equal
but opposite force on body A (the reaction)
Newton`s third law action –reaction force
i) Always occur in pair
ii) Always act on different bodies
iii) Always are the same type of force
iv) Always have the same magnitude
v) Always act in the same line
vi) Always act in opposite directions.
Examples of its application in include:
i) The propulsion of a rocket or jet engine
ii) The operation of a garden sprinkler as it rotates
iii) The force that a seat belt exerts on a passenger as the passenger is
jerked forward and exerts a force on the seatbelt in so doing.