Newton's Laws Session 2 POST - Part 1 PDF

Movement Science 1: Session II Newton’s Laws Adapted with permission from Dr. Cole, George Washington University Program in Physical Therapy,  Objectives Describe the terms and concepts that apply to forces, motion, levers,...

Movement Science 1: Session II Newton’s Laws Adapted with permission from Dr. Cole, George Washington University Program in Physical Therapy,  Objectives Describe the terms and concepts that apply to forces, motion, levers, and other biomechanical components Apply kinesiological principles of kinematics and biomechanics to movement and movement dysfunction Analyze kinetic and kinematic factors influencing human movement  Perspective Family Guy  Laws that Govern Motion Newton’s Laws! –1st Law: Law of inertia –2nd Law: Law of acceleration –3rd Law: Law of action-reaction Newton’s First Law: Law of Inertia 1st Law  Inertia First Law: Law of Inertia –Object at rest will stay at rest; objects in motion stay in (constant) motion Linear Motion: Force is required to start, stop, accelerate, decelerate motion Rotational Motion: Torque is required to start, stop, accelerate, decelerate motion  Inertia Inertia: Measure of an object’s resistance to a change in velocity (linear) Proportional to mass Units are the same as mass (e.g. Kg, though inertia is NOT a mass) – Moment of Inertia: Measure of an object’s resistance to a change in angular velocity Proportional to not only mass, but also the distribution of mass (m*r2)  Equilibrium Static Equilibrium: –linear and rotational velocities are zero (body not moving) –Net acting forces = 0: ∑ Fx= 0 ∑ Fy= 0 ∑ τ = 0 Dynamic Equilibrium: –linear or rotational velocities are constant –Net acting forces are still 0 (no acceleration) ∑ Fx = 0 ∑ Fy=0 ∑ τ = 0  https://youtu.be/EabUUrZFnFE Let’s consider the persons vertical velocity and acceleration during the following times – Standing in plane – First jump out – Getting going faster – Open parachute – Once you slow down Consider forces of gravity, air resistance, force from floor of plane  Mass moment of Inertia For linear motion force is equal to mass times acceleration – F=ma For rotational motion, torque (τ) is equal to mass moment of inertia (I) times angular acceleration (ω) –τ = I ω I is large, since I is small, since –Where I = m*r2 divers mass is far divers mass is close away from center of to the center of rotation. Angular rotation. Angular acceleration is acceleration is therefore slow. therefore fast.  Rotational Motion ω Who has a large I and small ω and who has a small I and large ω? Newton’s Second Law  Newton’s Second Law Second Law: –Describes behavior of objects when not in equilibrium! –When the SUM of forces acting on an object is nonzero it will accelerate in the direction of the of the net force.  Newton’s Second Law Acceleration is dependent upon two variables: –Net (Σ) force –Mass (linear) or mass moment of inertia (rotational) of the object ΣF = ma or Σ τ = I α Acceleration takes place in the same direction of the force/torque and is inversely proportion to mass/mass moment of inertia of the body  Newton's Second Law Force Acceleration Relationship –Cause effect relationship –Left side of equation = cause –Right side of equation = effect (static equilibrium) (static equilibrium) (linear acceleration) (angular acceleration) Demonstration Newton’s Third Law  Newton’s Third Law Third Law: –For every reaction, there is an equal and opposite reaction –When two objects interact – what each object experiences is dependent on its mass –Application: Ground reaction force (foot strikes the ground) Joint reaction force  Ground Reaction Force Ground reaction force: Force exerted by the ground on the body, that is equal in magnitude and opposite in direction to the force the body applies to the ground Ground Reaction Force!  Joint Reaction Force! 7N Force between the surfaces of the joint Develops “in reaction” to the other forces 5N JRF = usually the difference between the muscle force and external force JRF stops the forearm from accelerating upwards  Opposing forces: co-contraction Co-contraction is helpful! The quad pulls the tibia anteriorly The ACL prevents anterior translation of the tibia relative to the femur by pulling the tibia posteriorly Your body usually co-activates the hamstrings with the quads, because the hamstrings pull the tibia posteriorly, decreasing the force the ACL has to create (and withstand) FR Noyes (1976)  https://youtu.be/EabUUrZFnFE In plane – Vertical velocity is zero – Net force is zero, force from floor of plane equal and opposite of force of gravity – Static Equilibrium: First jump out – Gravity is greater than force of air resistance – Accelerate downwards Getting going faster – Faster you fall, greater the force from wind resistance, while gravity is constant – Force become equal and opposite and fall at constant velocity/zero acceleration – Dynamic equilibrium Open parachute – Force from air resistance becomes really large because of parachute, is now much greater than gravity – Velocity decreases Once you slow down – Air resistance decreases – Gravity and air resistance are equal and opposite again – Velocity is constant – Dynamic equilibrium  Rotational Motion ω Who has a large I and small ω (right) and who has a small I and large ω (left)?