Dynamics: Force, Motion, Work and Power

Dynamics

• Dynamics is the study of forces and their effects on motion

Inertia

• Inertia is the property of mass that resists changes in its state of motion
• The larger the mass, the greater the inertia

Newton's First Law of Motion

• A body remains at rest or continues in uniform motion until acted upon by an external net force

Work

• Work is done when a force acts on an object, overcomes inertia, and sets it in motion
• If the object does not move through a distance, the work done is zero
• Work (W) = Force (F) x Distance (S)
• The SI unit of work is the joule, equal to 1 Newton metre (Nm)
• In the Imperial system, work is measured in foot-pounds, which is the effort to raise one pound of mass by one foot

Power

• Power is the rate of doing work
• Time required to do work is not considered when determining the amount of work done, but power considers time
• The SI unit of power is the watt
• One watt is the power generated when one joule of work is done in one second
• In the Imperial system, power is expressed in foot/pounds per second
• One horsepower is equivalent to 550 foot/pounds per second and 746 Watts
• Power = force x distance / time

Energy

• Energy provides the capacity for work to be done and effect change
• The SI unit of energy is the joule
• Energy can neither be created nor destroyed, only changed from one form to another (law of conservation of energy)
• A car converts the chemical energy in petrol into mechanical energy, heat, and sound

Potential Energy

• Potential energy is the stored energy in a body due to its position, condition, or chemical nature
• Hydroelectric power uses the potential energy stored in a mass of water flowing downhill

Kinetic Energy

• Kinetic energy is the energy a body possesses because of its motion
• When a body held aloft falls, its potential energy is converted to kinetic energy

Total Energy

• The total energy does not change, but potential energy can be transformed into kinetic energy and vice-versa
• A falling mass has maximum potential energy at its highest elevation
• Halfway through its fall, the potential energy equals the kinetic energy
• The kinetic energy is maximum the instant the body strikes the floor
• Potential energy is zero when there is no distance left to fall

Friction

• Friction is the resistance to motion when objects roll or slide in contact with each other
• Minimizing friction is sought using lubricant in most industrial applications
• The coefficient of friction (μ) indicates the differences in friction between materials

Types of Friction

• Starting or Static: Overcoming initial resistance until breakaway
• Sliding: Resistance during steady motion
• Rolling: Single point contact resistance, less than sliding, requires some friction for grip

Heat

• Heat is a useful form of energy directly related to work and the use of engines
• Heat results due to friction

Efficiency

• The efficiency of machinery is the ratio of work output to worker energy input
• Friction primarily determines the efficiency of a machine
• Friction creates energy losses as heat, sound, and sometimes light
• Reducing friction occurs by lubrication or streamlining

Momentum

• Momentum is the product of inertia and the motion it already has, namely the tendency of a mass to resist change in its state of motion
• Two types of momentum: linear and angular

Linear Momentum

• Linear momentum measures the tendency of a moving body to continue in motion along a straight line
• Momentum is defined as the product of the mass and velocity of a body

Angular Momentum

• Angular momentum measures the tendency of a rotating body to continue spinning about an axis

Impulse

• Impulse results when a force affects the motion of an object
• Impulse is the amount of change in momentum; equal to the force multiplied by time (I = Ft)

Gyroscope

• A gyroscope is any rotating mass whose supporting platform or case can be turned in one or more planes around the rotor without changing the rotor's plane of rotation
• Fundamental characteristics are gyroscopic inertia (rigidity in space) and precession

Gyroscopic Rigidity

• This this property of any rotating mass to resist changes to its plane of rotation unless an external force causes a change

Precession

• Is the the change of the plane of rotation caused by an external force

Explore the concepts of dynamics, including inertia and Newton's First Law. Learn how force relates to work and power. Understand the formulas for calculating work and power, along with their units of measure.