Physics Answers - Part 1

(My answers on Quora)

What are the two forces that keep a pendulum swinging?

The two forces are Tension and the Gravity The former always acts along the radius of the circle (towards the circle center), the latter acts downward. In the case of the vertical pendulum - Tension and the radial component of gravity combine to give the object the necessary centripetal/radial acceleration (changes velocity direction). A gravity component provides for the tangential acceleration that speeds up or slows down the pendulum bob.

With respect to the conical pendulum the horizontal component of Tension supplies the centripetal force. The vertical component balances the force of gravity (weight).

Is friction always undesirable?

In some cases it is but in many cases it is exactly what we need. Without friction between your feet and the ground you wouldn’t be able to walk on your own and if you did manage to somehow start walking you wouldn’t be able to stop.

Anybody who has used Duct tape to make patchy repairs can further attest to the benefits of friction. Indeed if you nail two pieces of wood together you are making use of friction. Static friction prevents unnecessary motion and that is a big positive. Kinetic friction helps us in our braking action.

There are a myriad of other examples on the benefits of friction. Remember as well that when you travel in a moving object (such as a car) it is the friction between you and and that object that allows you to keep moving with that object.

Yes… Friction can have its downsides at times. However in other cases it RULES!!

Is momentum related to energy?

If an object has linear momentum (p) which is the product of its mass and velocity (p= mv..with direction) then by definition it has motion relative to an external inertial frame of reference. Consequently it has translational kinetic energy which is the product of one-half of its momentum magnitude multiplied by its speed or Ek(Translational) =1/2pv.

If an object has angular momentum (L) - product of its moment of inertia (I) and angular velocity (L = Iω with direction) then it has rotational kinetic energy. Rotational kinetic energy is equal to one-half the magnitude of the angular momentum multiplied by angular speed. EK(rotational = 1/2Lω)

Remember that momentum is a vector quantity. Energy is a scalar entity.

Note that these equations are Galilean/Newtonian simplifications. At extremely high velocities we need to make the necessary adjustments consistent with special relativity.

Why is the law of conservation of momentum important?

The Laws of Physics are what they are - plain and simple. If they were otherwise the universe would look markedly different and indeed if sentient life had existence in such a hypothetical universe (in some form or another) the question of importance would hinge on the specific local reality.

The importance we assign to the Conservation of Momentum (COM), or any other physics law for that matter, is a value judgement of how it impacts us. We need to walk, run, swim or jump so we rely on Newton’s Third Law (which is an alternative way of stating COM), hence we value it.

We make use of rocket and jet propulsion for travel so we elevate the importance of COM again. Action-Reaction forces and collision physics have played a vital role in planetary and stellar formation so we highlight its importance. After all what we believe life to be requires these structures to function.

However this needn’t be the case. It is conceivable that superstructures may arise from systems that don’t adhere to COM. In which case those working off an alternative ‘'Blogger' in a parallel world may be offering thanks to an obvious paradigm that subscribes to a non-COM truism.

From where does the air get the kinetic energy to drive a turbine?

It gets it from the sun’s electromagnetic wave radiation. This causes the air particles to move faster resulting in a increase in the average kinetic energy of the particles. It is the average kinetic energy that we associate with the temperature of the air.

Layers of air heat up to varying degrees producing a temperature gradient which provides the impetus for convection currents. These currents attempt to equalize the temperature differences via the bulk movement of matter. It is this bulk movement of air that we refer to as wind.

When the wind hits a turbine system it causes the blades to rotate thereby transferring the kinetic energy of the wind into the kinetic energy of the turbines.

It is this kinetic energy that drives a generator (spinning magnet/coil system) producing electricity via a process known as Electromagnetic Induction.

Is acceleration a change in speed?

This isn’t correct. Acceleration is the measure of the rate at which velocity changes. It is calculated by dividing the change in velocity over the time interval. The directional nature of velocity needs to be considered. Like velocity, acceleration is a vector quantity. Speed is a scalar. An object undergoes acceleration if either the magnitude of its velocity (essentially speed) changes OR its direction of velocity is not uniform.

An object undergoing uniform circular motion (UCM) for instance has a constant speed in that it covers the same distance during the same time period but it has a non-constant velocity. The direction of its velocity is constantly changing as a function of the Centripetal force (net force directed to the center of the circle). It therefore is accelerating with the acceleration vector pointing toward the circle center.