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Flashcards in Linear Kinetics Deck (13)
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Laws of Motion

Law I - Law of Inertia
Law II - Law of Acceleration
Law III - Law of Reaction


Law I – Law of Inertia

“Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it”
“uniform motion” = constant velocity
“forces impressed” = external forces
Inertia = objects resistance to change in state of motion


Law of Inertia Interpretation

Object at rest and no external forces acting on it → remains at rest

Object in motion and no external forces acting on it → continues moving at constant velocity in straight line

Object at rest and external forces do act on it → object remains at rest only if resultant external forces is zero (net external force = 0)



Important in impact situations
Momentum: “quantity of motion”
Any object which has both mass and a velocity is said to have momentum
Static objects have zero momentum
Momentum is a vector quantity

Product of a body’s mass and velocity

M = m•v

(units of measurement: kg.m/s


Conservation of Momentum

“The total momentum of any given system will remain constant unless acted upon by an external force.”
What is meant by a “system”?
Object (or group of objects) chosen for analysis
Isolated system: “Objects isolated from external forces”
Total amount of momentum in a system remains the same
Momentum before impact equals momentum after impact

(mv)1 = (mv)2


Law II – Law of Acceleration

“The change of motion is proportional to the force impressed and is made in the direction of the straight line in which that force is impressed”
“change of motion” = acceleration


Law II – Law of Acceleration Interpretation

If a net external force is applied to an object
→ object will accelerate in direction of net external force
→ acceleration will be proportional to net external force & inversely proportional to its mass


Measure of the Force (Law of Acceleration)

Measure of force is the Newton
1 Newton is the amount of force it takes to accelerate a 1- kg object 1 m/s2
N = 1 kg * 1 m/s2
N = kgm/s2

F = ma


What is impulse?

When force is applied to a body, the resulting motion is dependent upon both the magnitude and duration of force
↑ motion: ↑ magnitude or ↑ duration

Product of force and time over which force acts
Impulse = F·t
Units: N·s


Impulse (2)

Product of a force and the time interval over which the force acts
Impulse = F·t

Momentum changes rely on impulse:
Magnitude of external force
Length of time over which external force acts


Impulse-Momentum Relationship

a = Δv / Δt
∑F = ma


Law III – Law of Reaction

“To every action there is always opposed an equal reaction; or, the mutual action of two bodies upon each other are always equal and directed to contrary parts”
“Action” & “Reaction” = Force


Law III – Law of Reaction Interpretation

Forces never act in isolation, always in pairs
Forces are equal in magnitude
Forces are in opposite direction
Action (force) and Reaction (counterforce) act on different objects
Don’t cancel each other out
May have different effect on the objects