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 = 1kg•m/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: -9.81 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 = 1kg•m/s2
 1352 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 667.5 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) 667.5 N = 267 N
weight = mass gravity
667.5 N = m 9.81 m/s2 or 667.5 kg m / s2
9.81 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=3mX4m
 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?
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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 = 0.0004 kg/cm3
γ = wt/V
γ = 120 N/30,000 cm3
γ = 0.004 N/cm3