Chapter 3 Kinetic Concepts for Analyzing Human Motion Faizan Zaffar Kashoo MET-235

Basic Concepts Related to Kinetics What is mass? quantity of matter composing a body (dog, tree, desk, swimming pool, you) represented by m units are kg

Basic Concepts Related to Kinetics What is inertia? tendency to resist change in state of motion proportional to mass has no units

Basic Concepts Related to Kinetics Clearly, the weight bar will stay in place in the absence of being lifted because of it’s inertia.

Basic Concepts Related to Kinetics What is force? a push or a pull characterized by magnitude, direction, and point of application F = ma unit is the Newton (N)

Question: How much force must be applied to a 0.5 Kg ice hockey puck to give it an acceleration of 30 m/s2?

Solution: F = ma 1N = 1kgm/s2 F = 0.5 kg (30 m/s2) F = 15 N

Basic Concepts Related to Kinetics What is a free body diagram? (diagram showing vector representations of all forces acting on a defined system)

Basic Concepts Related to Kinetics What is a net force? the single resultant force derived from the vector composition of all the acting forces the force that determines the net effect of all acting forces on a body

Basic Concepts Related to Kinetics What is a torque (T)? the rotary effect of a force the angular equivalent of force also known as moment of force

Basic Concepts Related to Kinetics What is a torque? T = Fd (the product of force and the perpendicular distance from the force’s line of action to the axis of rotation)

Basic Concepts Related to Kinetics What is the center of gravity? point around which a body’s weight is equally balanced in all directions point that serves as an index of total body motion point at which the weight vector acts same as the center of mass

Basic Concepts Related to Kinetics The weights are balanced, creating equal torques on either side of the fulcrum (pivot).

Question: Two children sit on opposite sides of a playground seesaw. Joey, who weighs 220 N, sits 1.5 m from the axis of the seesaw, and Suzy, who weighs 200 N, sits 1.7 m from the axis of the seesaw. How much torque is created at the axis by each child? In which direction will the seesaw tip?

Solution: T = Fd┴ Tj = 220 N(1.5 m) Tj = 330 Nm Ts = 200 N(1.7 m) Ts = 340 Nm Ts > Tj Seesaw tips toward Suzy

Basic Concepts Related to Kinetics What is weight? attractive force that the earth exerts on a body wt. = mag (product of mass and the acceleration of gravity: m/s2)

Basic Concepts Related to Kinetics What is weight? the point of application of the weight force is a body’s center of gravity since weight is a force, units of weight are units of force: N

Question: William Perry, defensive tackle and part-time running back better known as The Refrigerator, weighed in at 1352 N during his 1985 rookie season with the Chicago Bears. What was Perry’s mass?

Solution: Wt = mag 1N = 1kgm/s N = m(9.81m/s2) m = 138 kg

Question: Gravitational force on planet X is 40% of that found on the earth. If a person weighs N on earth, what is the person's weight on planet X? What is the person's mass on the earth and on planet X? (weight on planet X = 267 N; mass = 68 kg on either planet) Solution: If gx = 40% gearth, weightx is 40% of weightearth. Wx = (0.4) Wearth = (0.4) N = 267 N weight = mass  gravity N = m  9.81 m/s2 or kg  m / s m/s2 m = 68.0 kg (mass will not change when gravity changes)

Basic Concepts Related to Kinetics What is pressure? force per unit of area over which the force acts commonly used to describe force distribution within a fluid (e.g. blood pressure, water pressure) units are N/m2

Basic Concepts Related to Kinetics What is stress? force per unit of area over which the force acts commonly used to describe force distribution within a solid units are N/m2

Basic Concepts Related to Kinetics What is stress?

Question: A gymnastics floor mat weighing 220 N has dimensions of 3 m X 4 m X 0.04 m. How much pressure is exerted by the mat against the floor? Solution: Area = Length X Width A = 3 m X 4 m A = 12 m2 p = F/A 1Pa = 1N/ m2 p = 220 N/12 m2 p = 18.3 Pa

Question: If 37% of body weight is distributed above the superior surface of the L5 intervertebral disc and the area of the superior surface of the disc is 25 cm2, how much pressure exerted on the disc is attributable to body weight for a 930 N man? (13.8 N/cm2) Solution: Weight of man's body above L5 disc = 0.37 X 930 N = 344 N Pressure on disc = F/A = 344 N / 25 cm2 = 13.8 N/cm2

Basic Concepts Related to Kinetics What is volume? space occupied by a body has three dimensions (width, height, and depth) units are m3 and cm3

Question: What is the volume of a milk crate with sides of 25 cm, 40 cm, and 30 cm? Solution: Volume = Length X Width X Height 1l = 1000 cm3 V = 25 cm X 40 cm X 30 cm V = 30,000 cm3 or 30 l

Basic Concepts Related to Kinetics What is density? mass per unit of volume represented with the small Greek letter rho: units are kg/m3

Basic Concepts Related to Kinetics What is specific weight? weight per unit of volume represented with the Greek letter gamma:  units are N/m3

Question : If the contents of the crate described in the previous question weigh 120 N, what are the average density and specific weight of the box and contents? Solution: m = wt/ag m = 120 N/9.81 m/s2 m = 12.2 kg ρ = m/V ρ = 12.2 kg/30,000 cm3 ρ = kg/cm3 γ = wt/V γ = 120 N/30,000 cm3 γ = N/cm3