Linear and Angular Kinetics in Mechanics

Kinetics

The branch of mechanics that deals with the causes of motion.

Force

A push or pull that tends to change the current state of motion.

Magnitude

The amount of force applied.

Direction

The pushing or pulling force represented by arrows.

Newtons

The unit in which force is calculated.

Non-contact forces

Forces that do not require physical contact, such as gravitational force.

Contact forces

Forces that require physical contact, such as ground reaction force, joint reaction force, friction, fluid resistance.

Ground Reaction Force (GRF)

The force exerted by the ground on a body in contact with it (equal and opposite).

Friction Force

Static prevents an object from starting to move, dynamic opposes the motion of an object that is already moving.

Free Body Diagram

A diagram that shows all the forces acting on an object.

Static Free Body Diagram

A diagram where forces are equal on both sides, such as in a tug of war.

Dynamic Free Body Diagram

A diagram that represents movement, such as running away.

Newton's 1st Law of Motion

An object remains in motion unless acted upon by an outside force.

Newton's 2nd Law of Motion

Change of motion occurs in a straight line in the direction the force is applied (F = ma).

Newton's 3rd Law of Motion

For every action, there is always an equal and opposite reaction.

Impulse

The change in momentum, calculated as force multiplied by change in time.

Momentum (P)

The product of mass and velocity.

Impulse-Momentum Relationship

Force x change in time = mass x change in velocity.

Inertia

The property of an object to resist changes in its state of motion.

Application to sport and exercise

Understanding how Newton's laws apply to physical activities.

Static vs Dynamic Friction

Static friction prevents motion, while dynamic friction opposes motion of an already moving object.

Work

Product of force and the amount of displacement in the direction of that force

Positive Work

Force acting on object is in same direction as displacement

Negative Work

Force acting in opposite direction of displacement

Zero Work

Can be 0 if up and down are the same

Units of Work

joule 1j = 1Nm

Work Formula

U = F x displacement

Power

Rate of doing work in a specific amount of time

Power Formula

P = work/ change in time to do the work

Average Power Formula

P = average of Force x average of Velocity

Units of Power

watts (W)

Kinetic Energy (KE)

Energy due to motion, affected by the mass and velocity of object

Kinetic Energy Formula

KE = ½ MV^2

Units of Kinetic Energy

kg(m^2/s^2) or joules

Potential Energy (PE)

Capacity to do work

Gravitational Potential Energy

PE due to object's position relative to earth

Gravitational PE Formula

Gravitational PE = weight x height

Strain Potential Energy

PE due to an object's deformation

Strain PE Formula

Strain PE = 1/2 x spring constant x change in x^2

Angular Kinetics

Deals with causes of motion and forces that lead to a rotation around an axis

Torque

Product of force and the perpendicular distance to its line of action (moment arm)

Torque Formula

T = F x r

Center of Mass

Balancing point of the body, where torque equals 0

First Class Lever

Force on one side resistance on other of lever, ex. Seesaw, agonist and antagonist muscle

Second Class Lever

Force and resistance force on same side of fulcrum, rare in human body

Third Class Lever

Force and resistance on same side, force acts between fulcrum and resistance force, requires large amount of effort