Linear kinetics: Newtons laws

the center of gravity (COG) can also be described as

-Point that serves as an index of total body motion - Point at which the weight vector acts - Same as the center of mass

Newtons second law: law of acceleration

A force applied to a body causes acceleration of that body • The magnitude of the acceleration will be proportional to the force • The acceleration will occur in the direction of the net force • The acceleration will be inversely proportional to the body's mass, ΣF = m a, a = ΣF/m

Newton's fourth law: law of gravitation

All bodies are attracted to one another with a force proportional to the product of the masses and inversely proportional to the square of the distance between them Fg = G • m1 • m2/d^2 the gravitational constant (6.67×10-11 N⋅m2/kg2), however, the whole equation of (G x m1/d^2) is equal to the -9.8m/s^2 -Therefore, on earth: Fg = -9.81 m/s2 • m2 (m2 ranges for each body or object)

law of inertia

Another name for Newton's First Law

Better sprinters can generate a more forward-directed horizontal component of the resultant GRF than less skilled sprinters because?

Faster top running speeds are achieved with greater ground forces not more rapid leg movements

Newton's third law of motion

For every action there is an equal and opposite reaction -When one body exerts a force on a second body, the second body exerts a reaction force on the first body that is equal in magnitude and opposite in direction

ground reaction force

GRF = Force made by the ground in reaction to a force made by a body on the ground -Ground reaction force produces an acceleration of the body in the direction of the reaction force -But the force of the body on the ground does not produce an acceleration of the ground

Example of Newtons first law: A barbell at rest on the floor has a mass of 20 kgoWbarbell = 20 kg * -9.81 m/s2 = -196.2 N. You apply a upward force of +170 N. Will the barbell move?

No because the applied force is not greater than 196.2N

Remember that acceleration =

Sum of forces/mass

if a body is at rest it will remain at rest until

a net force acts on it

weight is a force and so it is also

a vector quantity, and its center of gravity is equal to its point of application on the body or object

Any time an object starts, stops,speeds up, slows down, or changes direction, it is

accelerating and a net external force is acting to cause this acceleration

momentum is

amount of motion of an object • Amount of motion = product of mass and velocity

what is force?

any interaction between two objects or bodies that tend to cause an acceleration (a push or a pull), a force attempts to accelerate a body by overcoming its inertia -∑Force = mass*acceleration

Golfers switch a large GRF from the back leg during

backswing to the front leg during downswing-where the force is applied dictates the direction of the components

Newton's first law: If an object is in motion and the net external force acting on it is zero, the object will

continue moving at constant velocity in the same direction

The GRF can be resolved into horizontal (FH) and vertical (FV) components. The use of a longer stride length during running increases the retarding horizontal component FH, what is meant by this

depending on where the foot is placed will dictate how much force is directed in the vertical vs horizontal direction, so in this slide a longer stride length or step length increases the horizontal component of force

Newton's first law of motion

essentially object will remain at rest or at constant velocity unless an external force comes in to overcome the inertial force and change the object's motion

the reaction force is the

exerted back at the person or object in response (i.e. reaction) to all the applied forces • Vertical: 'Normal' reaction force • Horizontal: frictional force

pressure

force per unit of area over which the force acts- Represented by p = F/A - Commonly used to describe force distribution within a fluid (e.g. blood pressure, water pressure) -may ask a question about the stress of something but he means the pressure - Units are N/cm2 or N/m2

examples of forces that can change an objects motion in regards to Newton's first law

frictional force and air resistance

High jumpers use the GRF to convert a

high horizontal approach velocity to an upwardly directed vertical velocity- where the force is applied dictates the direction of the components

GRF can be resolved into

horizontal and vertical components, the use of a longer stride length during running increases the retarding horizontal component

characteristics of force include

magnitude, direction, point of application, line of action -all of these characteristics determine how much force is exerted and or how much motion is changed

inertia is directly proportional to

mass -object with large mass= lots of inertia -object with small mass= small amount of inertia

density

mass per unit of volume • Represented with the small Greek letter rho: ρ • Units are kg/m3

center of gravity (COG)

point around which the body's weight is equally balanced in all directions, AKA all forces and torques around the body's weight are all equal

what is mass?

quantity of matter composing a body or an object, Units (kg) and it is a measure of an objects inertia -always constant

Runners are classified as

rearfoot, midfoot, or forefoot strikers depending on what part of the foot first hits the ground.

Newtons first law: if an object is at rest and the net external force acting on it is zero, the object must

remain at rest

volume

space occupied by a body,length x width x height. - Represented by v = l x w x h- Units are cubic centimeters cm^3

Newtons first law of motion is essentially a

static scenario • V = 0: a body at rest will tend to stay at rest • V = #: a body in motion will tend to remain in a constant state of motion -this is always true unless an external force acts on the body to cause a change in motion

kinetics is a branch of dynamics that deals with

the causes of motion -involves the study of forces acting on objects and the effects of the forces on the objects -quantities include mass, inertia, force, momentum, impulse, energy

acceleration will take place in

the direction of the net force

Baseball pitchers use a GRF of 35% BW in

the direction of the pitch with the push-off leg- this is just saying that where the force is applied dictates the direction of the components

what is weight?

the force of gravity on an object or body, units (N) and it is equal to mg (mass times the acceleration due to gravity) -not constant, it is constantly changing due to gravity

Sir Isaac Newton discovered

the fundamental relationships of modern mechanics which highlights the inter-relationships between the basic kinetic quantities -First Law - Law of inertia - Second Law - Law of acceleration - Third Law - Law of reaction - Law of Gravitation

The GRF will vary based on

the gait pattern of the runner and also the magnitude of the vertical component of GRF is usually two to three times the runner's body weight.

if you were asked how much inertia an object has you would give

the mass

impulse

the product of force and the time over which the force acts (Ft) - Units are Newton seconds (N • s)

how are newtons laws relevant?

they describe how kinetics causes kinematics (how forces cause motion to occur)

When a ball is thrown, kicked or struck with an implement: Its speed will be in direct proportion to the striking force and indirect proportion to the ball's mass, what is meant by this?

this basically just means that acceleration and force are inversely proportional to mass, you can see these relationships by rearranging the equations (NOTE: if mass is in the denominator compared to acceleration or force, then it's inversely proportional)

In the collision of two moving bodies, the resultant is a function of the direction and speed of the initial body and the direction and magnitude of the force of the colliding body, what is meant by this

this is talking about Newton's second law, acceleration of the colliding bodies will move in the direction of the net force

mathematically newton's first law is modeled as

v = constant if ΣF = 0 or ΣF = 0 if v = constant -where: v = instantaneous velocity and ΣF = net force

free body diagrams are used to

visually determine the effect of forces acting on an object• i.e. how they will accelerate

Since Newton's first law applies to components of motion you can have the following equations modeled as such

vx = constant if ΣFx = 0 or ΣFx = 0 if vx = constant vy = constant if ΣFy = 0 or ΣFy = 0 if vy = constant vz = constant if ΣFz = 0 or ΣFz = 0 if vz = constant

specific weight

weight per unit of volume - Represented with the Greek letter gamma:γ - Units are N/m3

Newton's first law: If an object is at rest, the net external force acting on it is

zero

Newton's first law: if an object is in motion at constant velocity in a given direction, the net external force acting on the object is

zero

you can rearrange the newtons second law force equation for examine the effects of force

∑F = m(vf - vi)/Δt

the net force is

The combination of all forces acting on an object and it is the single resultant force derived from the vector composition of all the forces acting on the object -this force will determine the net effect of all the forces acting on the body

inertia is

The tendency of an object to resist a change in motion -if an object is in motion then the object will resist a change in that motion -if an object is at rest then the object will resist the change to start moving

free body diagrams

a diagram showing all the forces acting on an object -object of interest is isolated (ie. a body part, assistive device etc) -all forces acting on object are shown as vectors