1. ESS 303 – Biomechanics Linear Kinetics

2. Kinetics The study of the forces that act on or influence movement Force = Mass * Acceleration: F = M * a Force is a vector (magnitude & direction) SI units for force are Newtons; (kg * a)

3. Newton’s 3 Laws of Motion Inertia: the resistance of an object to changes in motion; an object at rest tends to stay at rest unless acted upon; an object in motion tends to stay in motion unless acted upon Acceleration: change in motion is proportional to the force impressed; the direction of the change in motion is in the direction of the force impressed Action/Reaction: Objects “push back” with equal force and in the opposite direction as the force impressed (equal and opposite)

4. Linear Kinetics Concurrent forces: forces that act on the same point at the same time Colinear forces: forces in a straight line (calculate the sum) Coplanar forces: forces in a plane (connect the vectors and calculate the displacement 5N + 7N – 10N = 2N +=

5. More Terms Linear Momentum: Quantity of motion, a product of an object’s mass and velocity Impulse: Product of force and time, changes the momentum of a system Impact: Collision, typically a large force exchange over a small time period Perfectly elastic impact: All velocity is conserved Perfectly plastic impact: All velocity is lost Coefficient of restitution: Elasticity reference for colliding bodies

6. Impacts Elastic Plastic

7. Contact Forces Ground reaction force Joint reaction force Friction Fluid resistance (concept of laminar flow) Inertial force Muscular force Elastic force

8. Contact Forces (GRF)

9. Free Body Diagram Force of gravity (weight of segment) Muscle Force (Line of Application) Joint Reaction Force Angle of Pull Point of Application

10. Force of Friction Coefficient of friction (μ) Static friction Dynamic friction Direction of Motion F friction F pull F weight Friction Force Applied Force Static Dynamic Instant of Motion

11. Linear Equations Pressure: p = F / A; 1 Pascal = 1N/m 2 Work: W = F * Distance, Joules (J) Power: P = (W / t) = (F * Velocity); Watts Weight = Mass * Acceleration

12. Linear Kinetics Problems Calculate weight for a mass of 72kg? F = (M * a) = (72kg * 9.81m/s 2 ) = 706.32 N How much force is needed to accelerate 500N to 6.5 m/s 2 ? 500N / 9.81m/s 2 = 50.97kg F = (M * a) = (50.97kg * 6.5 m/s 2 ) = 331.29N Calculate pressure if 500N are applied over an 11m 2 area? p = (F / a) = (500N / 11m 2 ) = 45.45 Pascals