Lecture 4: Linear Kinetics

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Lecture 4 Linear Kinetics
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Inertia

• The tendency to keep the current state of motion
  (velocity)
• Difficulty in changing the state of motion
• Determines the physical personality of the object
• No inertia no existence

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Mass

• Quantity of matter an object possess
• Determines the cause and effect relationships in
  the linear motion
• Measure of the linear inertia (inertia in the
  linear motion)
• Unit kg

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Force

• Push or pull that can cause a change in the state
  of motion
• Cause of motion
• F gt R ? motion
• Unit N (Newton) kgm/s2
• Properties
• Magnitude direction
• Point of application

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Internal vs. External Force

• Internal inside the mechanical system
• External outside the mechanical system
• External internal forces

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Newtons 1st Law of Motion

• Law of inertia
• Unless acted upon an external force, the state of
  motion remains unchanged

Static Inertia
Dynamic Inertia
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Newtons 2nd Law of Motion

• Law of acceleration
• An external force acting on an object causes
  acceleration of the object
• The acceleration is proportional to the force,
  inversely proportional to the mass (inertia)

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• Resistance net force

• No external force 1st law

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• Only external forces can cause acceleration of
  the CM

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Newtons Law of Gravitation

• any two particles of matter attract one another
  (gravitation)
• proportional to the product of their masses and
  inversely proportional to the square of the
  distance

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• Weight
• gravitation force exerted by the earth
• (mass) (gravitational acceleration)

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Newtons 3rd Law of Motion

• Law of reaction
• to every action, there is an equal and opposite
  reaction
• bi-lateral nature of force exertion
• same magnitude, opposite direction

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Collision in ice hockey m1 90 kg, m2 80 kg,
F1 450 N F2 ?
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Ground Reaction Force

• reaction supplied by the ground

• due to contact with the ground
• roles
• braking force
• propulsive force

F R
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• Measurement force plate

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• Walking

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• Running

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• Golf
• Weight shift
• 2 plates

• Archery shooting
• Stability balance
• Center of pressure pattern
• Area
• Complexity

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Friction

• Force due to relative contact

• Passive force (resistance)
• Direction opposite that of motion or motion
  tendency

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• Net force motion

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• Sliding vs. rolling
• sliding due to relative motion of the surfaces
• rolling due to temporary deformation of the
  surfaces

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• Strategies
• minimize friction as a resistance
• lubrication - synovial fluid
• tire inflation paving
• skating - gliding
• maximize friction as a source of propulsion
• cleats
• skating - pushoff

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Pressure

• Amount of force acting over a unit area

• Unit Pa (Pascal) N/m2
• Safety injury
• Reduce force first
• Increase area

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Momentum

• Amount of motion

• How hard it is to stop an object
• Capacity to give impact (impulse)
• Vector
• Unit kgm/s

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• Total momentum of a system

m1 90 kg v1 6 m/s
m2 80 kg v2 7 m/s
M (90)(6) (80)(-7) 540 560 -20 kgm/s
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• Principle of momentum conservation
• No external force

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Mbefore 540 (-560) -20 kgm/s
Mafter (90 80)vafter 170 vafter
170 vafter -20 kgm/s
vafter -20 / 170 -0.12 m/s
player 2 pushes 1 at 0.12 m/s
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Impulse

• Accumulated effect of force exertion

• Vector
• Unit Ns

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• Causes change of momentum

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• Strategies
• Giving impulse
• batting, throwing, etc.
• Maximize impulse
• Receiving impulse
• landing, receiving, etc.
• reduce impulse
• reduce impulse force

• reduce pressure

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Impact

• Coefficient of restitution

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• Elasticity
• Ability to regain the original shape after
  deformation
• Types
• Elastic e 1
• Inelastic 0 lt e lt 1
• Plastic e 0

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• Ball drop

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• Direct vs. oblique impact
• Angle of incidence vs. angle of reflection
• Spin friction

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Mechanical Work

• Accumulated effect of force over displacement

• Scalar
• Unit J (Joule) Nm

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• Positive vs. negative work

• Positive concentric muscle activation
• Negative eccentric muscle activation

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Mechanical Power

• Rate of work

• Intensity of work
• Scalar
• Unit W (Watt) J/s

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Mechanical Energy

• Capacity to do mechanical work
• Kinetic energy due to motion
• Potential energy due to position
• ME KE PE

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• Mechanical energy conservation
• Gravity is the only external force

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• Work-energy relationship

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