Flashcards in Linear Kinetics Deck (13)

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1

## Laws of Motion

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Law I - Law of Inertia

Law II - Law of Acceleration

Law III - Law of Reaction

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## Law I – Law of Inertia

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“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

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## Law of Inertia Interpretation

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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)

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## Momentum

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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

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## Conservation of Momentum

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“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

or

Momentum before impact equals momentum after impact

(mv)1 = (mv)2

m1=m2

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## Law II – Law of Acceleration

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“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

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## Law II – Law of Acceleration Interpretation

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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

8

## Measure of the Force (Law of Acceleration)

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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

a=F/a

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## What is impulse?

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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

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## Impulse (2)

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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

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## Impulse-Momentum Relationship

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a = Δv / Δt

∑F = ma

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## Law III – Law of Reaction

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“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

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