Dynamics: Inertia, Work, Power, Energy, Friction - Physics Notes PDF

TOPIC 2.3: DYNAMICS Inertia Inertia is the property of a mass which causes it to **resist any change in its state of** **Motion** **Newton's first law of motion states:** A body will remain at rest or continue its uniform motion in a straight line until acted upon by an external net force. Th...

TOPIC 2.3: DYNAMICS Inertia Inertia is the property of a mass which causes it to **resist any change in its state of** **Motion** **Newton's first law of motion states:** A body will remain at rest or continue its uniform motion in a straight line until acted upon by an external net force. The larger the mass, the greater the inertia. **WORK** **When a force acts on an object, overcomes inertia, and sets it in motion, work is** **done. Unless the object moves through a distance the work done is said to be zero.** **W=FS F = Force S = Distance** **The unit of work in the SI system is the joule, which equals 1 Newton metre (Nm)** **In the Imperial system of measurement, a measure of work is the foot-pound, the** **effort of raising one pound of mass by one foot.** **POWER** **Power is the rate of doing work. When determining the amount of work done, the time** **required to do the work is not considered. Power on the other hand takes time into** **consideration.** **The unit 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 of measurement, power is expressed in foot/pounds per** **second and 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.** **An important concept when thinking about energy is the law of the conservation of** **energy which states:** **Energy can neither be created nor destroyed. It can only be changed from one form** **to another.** **For example, a car turns the chemical energy found in petrol into mechanical energy,** **heat and sound.** **Potential Energy** **The potential energy in a body or of a body means stored energy, stored in the body** **because of its position, condition or chemical nature.** **Hydro electric power uses the energy stored by a mass of water flowing downhill.** **Kinetic Energy** **Kinetic energy is energy a body has because of its motion. If a body is held aloft and then released, as it starts to fall to ground the potential energy is converted to kinetic energy.** **Total Energy** **In accordance with the law of conservation of 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 highest elevation** **Once the mass is released and starts falling, the potential energy starts to be converted to kinetic energy. Halfway through its fall, the potential energy exactly equals the kinetic energy. Then, at the instant the body strikes the floor, the kinetic energy is maximum.** **It has no distance left to fall so potential energy is zero.** **FRICTION** **When objects move they usually roll or slide in contact with other objects or substances. Such sliding or rolling contacts have resistance to the force that causes the motion. This resistance is called friction.** **In most industrial applications the minimisation of friction is sought, with lubricant.** **The coefficient of friction refers to the differences in friction between various** **materials.** **The higher the coefficient of friction (μ), the greater the resistance between two** **surfaces.** **There are three types of friction:** **1. Starting or Static - Overcoming initial resistance until breakaway occurs.** **2. Sliding - Resistance during steady motion.** **3. Rolling - Single point contact resistance is less than sliding. Still need some friction otherwise the wheel will not grip.** **HEAT** **Heat is one of the most useful forms of energy because of its direct relationship with** **work, and with the use of engines** **Heat is also found as a consequence of friction.** **EFFICIENCY** **With any machinery, the efficiency is the ratio of work output to worker energy input.** **It is friction that primarily determines the efficiency of a machine, because the friction between moving parts creates heat, sound and sometimes light.** **All of these are classified as energy losses.** **Reducing friction is usually accomplished by lubrication or streamlining.** **MOMENTUM** **Inertia has been defined as the tendency of a mass to resist changes in its state of** **motion.** **Momentum however is the product of this inertia and the** **motion it already has.** **There are two types of momentum, linear and angular.** **Linear momentum is a measure of 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 is a measure of the tendency of a rotating body to continue to spin about an axis.** **IMPULSE** **f a force is applied to a moving body, that body's state of** **motion is altered.** **The momentum of the body is changed by an amount called** **the Impulse.** **Impulse I = Ft Force multiplied by time** **A SIMPLE GYROSCOPE** **A gyroscope is any rotating mass. A useful example is the type consisting of a rotor** **mounted on gimbals, so that its supporting platform or case can be turned in one or** **more planes around the rotor without changing the rotor's plane of rotation.** **Like all rotating masses, the gyroscope has two fundamental characteristics. These are gyroscopic inertia (rigidity in space) and precession.** **Gyroscopic rigidity** **This the natural property of any rotating mass to resist changes to its plane of rotation unless an external force causes a change.** **This is the reason a spinning top or coin remains upright until it runs down.** **Precession** **This the change of the plane of rotation caused by an external force.**